The PTSD prevalence in the total sample of 21.3% is quite high compared to findings from previous epidemiological studies [1, 6]. However, the result from the present study is based on a large number of convenience samples that have been shown to result in increased prevalence rates for PTSD compared to epidemiological studies . The PTSD prevalence from the trauma sample in the present study does not show a significantly different PTSD prevalence (21.9%) from that found in the total sample. Either the difference in PTSD prevalence between epidemiological and convenience samples is not as distinct as previously assumed or more likely, it is due to the overlap between the total sample and the trauma sample. However, the actual prevalence percentages in the samples are not of key interest as the variation due to gender and age on PTSD prevalence is the objective of the present study. Here, the distribution of epidemiological or convenience samples and the large number of the latter are not likely to affect the results to the same extent. It has been suggested that the gender difference in PTSD prevalence is approximately the same for epidemiological samples and convenience samples .
A 2:1 female/male PTSD ratio was found for both the total sample and the trauma sample, which is consistent with the well established finding of an approximately twofold higher PTSD prevalence among women compared to men [1, 6]. In the total sample the overall PTSD prevalence for men is 13% and for women it is 27.4%. However, the female/male PTSD ratio showed some fluctuation between age groups. It also showed variation from the total sample to the trauma sample. The highest female/male PTSD ratio was 3:1 in both samples but the highest ratio was found for a different age group in the total sample (21 to 25 years) than in the trauma sample (61 to 65 years). The age group of 71 to 75 years showed the lowest female/male PTSD ratio (1:1.5) in both samples.
Women were found to score higher on the HTQ in both samples. These findings are consistent with previous findings that also pointed out gender differences for the HTQ . The gender difference in the mean scores of the HTQ is highest for the age groups of 21 to 25 years and 51 to 55 years and smallest for the 71 to 75 years. The results from the present study show that men peak in total HTQ scores a decade sooner than women (41 to 45 years and 51 to 55 years, respectively). Additionally, both men and women seem to be more resistant towards PTSD at old age than earlier in their lives, which is consistent with some previous findings  but inconsistent with others [18, 19].
Some arguments have been made that the increased PTSD prevalence among women is due to a report bias because men tend to under-report and women tend to over-report symptoms of PTSD . Some of the variance has also been suggested to be due to the social expectancy related to the male and female gender role. Where women are expected to be vulnerable, men are expected to be tough and more resilient to trauma . In relation to the lifespan distribution of PTSD it is possible that some of the noticeable features in the prevalence of PTSD are caused by gender roles, life course expectations, or neurobiological developmental changes as well as by variations in trauma exposure.
PTSD prevalence and young age
Adolescence has been described as being concerned with identity formation and with the task of developing a sense of self-continuity [28, 29], which could contribute to the effects seen in the female and male patterns regarding the age groups of 13 to 15 year-olds, and 16 to 20 year-olds. Both the increased starting point for the early adolescents, as well as the following decrease in PTSD vulnerability for late adolescence, may, to some extent, be caused by identity-related issues.
The early 20s are for women characterised by an increased HTQ score. This is consistent with previous trauma and PTSD-related findings [1, 7] that demonstrated an increased risk of PTSD among women in their late teens and early 20s compared to those women at younger age. A long period of adulthood from the 20s to the 40s seems to be characterised by a relatively stable level of HTQ scores which indicates that the vulnerability to PTSD is present and somewhat constant for adult women, despite the fact that this period in life is known to hold many life changing moments such as, getting married, starting a family, choosing a career, and so on. Perhaps herein is a great part of the explanation. Frequent changes and individual development happening in tune with the modern female gender role throughout most of the period brings meaning and life satisfaction to each individual woman. However, the vulnerability rises to its peak around the early 50s where the risk of PTSD is significantly high. This deviates from the previous level of HTQ scores and hereby indicates a significant change in the life course caused by neurobiological or other factors.
Fluctuations in the reproductive hormones across menstrual phase and reproductive state in women have been found to influence the sympathetic system reactivity . An increased level of activity in the sympathetic or noradrenergic systems has additionally been found to be present in men and women with PTSD. It is, therefore, plausible that exposure to traumatic stress during different phases of the menstrual or reproductive cycles could influence the vulnerability to PTSD due to different effects at a neurobiological level. Menopausal women have shown increased cardiovascular and epinephrine responses to mental stress compared to premenopausal women , and PTSD symptoms have been associated with ambulatory cardiovascular functioning in postmenopausal women . This might provide a neurobiological explanation for the increased HTQ scores found for women at the age of 51 to 55 years in the present study. The age of 51 to 55 years is equivalent to the age of menopause. Changes in reproductive ability, hormonal levels, and sympathetic responses are some of the likely changes that happen along the transition from a premenopausal to a postmenopausal woman. The changes are, therefore, not merely neurobiological but also involve potential changes in self-perception, social participation, world beliefs, and adaptation to social gender roles. These changes might also add to an increased stress level or a greater vulnerability to PTSD.
For men a different pattern is seen in adulthood. The male pattern is characterised by a steady almost linear increase in HTQ scores, which begins in the late teens or early 20s and lasts to the early 40s, where the HTQ scores for men peaks. It is conceivable that the gradual rise in the risk of PTSD happens concordantly with a gradual change in the male gender role from being free and able to do as they please to being tied up with work and family responsibilities, resulting in a life with less autonomy and potentially more stress. The phenomena of male midlife crisis might, to some extent, influence the results in HTQ scores found in men.
PTSD prevalence in old age
A distinct decrease in HTQ scores is seen for women after the 50s and the lowest level is found for women in their late 60s or early 70s. For men a decrease in HTQ scores is also seen towards old age. After the 40s men show a gradual fall in the risk of PTSD. The lowest potential risk of PTSD is thus found for men in their late 60s. Old age has been considered to deal with the acceptance of earlier experiences in life and the fact that death is more imminent than earlier . According to Erikson  old age is concerned with the psychosocial crisis of ego integrity versus despair. If the crisis is resolved favourably, ego integrity, wisdom, and life satisfaction is reached [33, 34]. This could in fact be part of the reason for the decreased risk of PTSD seen in the 50s and 60s for men and in the late 50s, 60s, and early 70s for women. Some suggestions have additionally been made that a decline in self-occupation, an increase in time spent in quiet reflection, and a decreased interest in superfluous social interactions also are characteristic of old age . Satisfaction with the life led, wisdom in retrospect, and the acceptance of a forthcoming death without fear may very likely affect coping strategies and resilience to PTSD in a positive way. However, this is challenged by the results for both men and women in the present study. Thus, the risk of PTSD shows a small linear increase from the late 60s to the late 70s for men and from the mid to late 70s for women. It has been suggested that reaching the age of 80 or more involves special challenges and perhaps a new stage in psychosocial development [33, 34]. If this is the case the vulnerability to PTSD might also be different and involve special issues at such an old age. The effect might, to a limited degree, be detectable in the results for the 70 or 80-year-olds in the present study and thus explain the final rise seen in the HTQ scores.
Comparison with previous studies
Kessler et al.  found that the age group of 45 to 54-year-olds showed the highest risk of PTSD among men. Among women they found it to be between the age of 25 and 34 years. Creamer and Parslow  found the highest risk of PTSD to be present between the age of 18 and 24 years for both men and women. When converting our results into comparable 10-year-span age groups the highest risk of PTSD was found between the age of 45 and 54 years among men, and 55 and 64 years among women. The results for men are congruent with the findings by Kessler et al. The results for women, however, show a 30-year difference in PTSD peaks between the two studies. However, the age group of 55 to 64-year-olds was not included in the Kessler et al. study. The inconsistency with the findings of Creamer and Parslow could be due to methodological or cultural differences.
The findings from the present study describe the effect of gender and age on PTSD prevalence in a Nordic country culture context. The results for effect of age on PTSD prevalence resemble the results found by Norris et al.  in relation to an American culture context. The total picture of PTSD prevalence (based on HTQ mean score) associated with age shows that the prevalence of PTSD follows a curvilinear picture where middle-aged participants show a higher degree of PTSD than young participants, who again show a higher level of PTSD than older people. The results, thereby, are somewhat similar to the ones found in the US by Norris et al. and might reflect how PTSD appears in Western cultures. In contrast, Maercker et al.  found a higher prevalence of PTSD among participants above the age of 60 years in Germany. This might show that cultural comparison of PTSD prevalence or other psychological measures can be delicate due to, for example, historical, economical, or political reasons.
Age group ranking
Both the HTQ mean scores as well as the categorical PTSD scores can be seen as a way to estimate the potential risk of PTSD or the vulnerability to PTSD. The dimensional and categorical results of PTSD were both ranked by age groups in order to find the estimated distribution of PTSD prevalence according to age. The results show differences in the rank of age groups due to a dimensionally (HTQ) or a categorically (PTSD%) estimated PTSD prevalence. The rankings of age groups can be seen in Table 5. The highest rank for women by both the PTSD percentages and the HTQ scores is found for women at the age of 51 to 55 years. For men the highest rank for both HTQ scores and PTSD percentages are identical as the age group of the 41 to 45-year-olds is found to top both ranking lists for men. The second and third most PTSD vulnerable age groups according to the HTQ are the 31 to 35 years sample and the 13 to 15 years sample among men and the 56 to 60 years sample and the 13 to 15 years sample among women. For the categorical PTSD prevalence the second and third rank are found for the 46 to 50 years sample and the 51 to 55 years sample among men and the 56 to 60 years and the 21 to 25 years samples among women. Thus, the two types of measurement primarily show differences in the ranking of age groups but some congruent results are found. Recent articles indicate that the future choice of measurement in the DSM will favour the dimensional proportions [36, 37]. As indicated by the results from the present study differences are seen between the two types of measurement. Which is preferable to the other is not settled by the present study, but the HTQ score does withhold more information. Thus, the results based on the dimensional approach might be more differentiated. This might add some consideration to the ongoing discussion of the preference of dimensional models rather than categorical measures in the research agenda for the DSM-V .
Limitations of the study
In this study, 25 different studies were included to test the hypothesis that men and women show a difference in age distribution of PTSD prevalence. Possible limitations due to a lack of representativeness in the samples, undetected cohort effects, and biases due to method failure are likely to have influenced the results. However, a great strength of the study is the size of the population by which each age group has reached a certain representative size. All the data has been analysed by retrospective analysis and no contact has been made with any participants in the process of the present study. Possible reporting biases could, therefore, have gone undetected or have been deleted due to ambiguity. A large part (76%) of the total sample consisted of convenience samples. This of course must be viewed as a potential limitation of the study and must be taken into account when interpreting the results of PTSD prevalence.
Another limitation of the study concerns cultural considerations. The present study is based on data gathered in the Nordic countries of Denmark, Iceland, and The Faroe Islands. The results, therefore, must be taken with some consideration when comparing to other countries or areas outside the Nordic region. Norris et al.  clearly showed that cultural differences are found in the PTSD prevalence rates. Therefore, it is likely that cultural considerations account for some of the variance seen in the present study. It is desirable that the combined effect of gender and age on PTSD is studied in other parts of the world in order to see if the present study has created a precedent for the combined effect of gender and age on PTSD or for the lifespan distribution of PTSD.
To conclude on the matter of gender differences in the lifespan distribution of PTSD it would be beneficial to compare the age distribution of PTSD prevalence with the age distribution of trauma exposure in order to find potential discrepancies and in order to clarify the true extent of the vulnerability or risk of PTSD. If controlling for trauma exposure does not indicate that the combined gender and age effect on PTSD is due to increased trauma exposure at certain periods in the male or female life course then the results from the present study demand further research. Thus, the goal of future research would be to verify the presented findings as well as to find possible explanations for these findings. Future research should focus on the construction of usable and representative age groups with inclusion of not only young and adults but also older people in order to describe the entire lifespan of PTSD distribution. The inclusion of participants beyond the age of 80 would touch on something new and concurrently bring diversity into the range of the population examined. Future research should also include an examination of the association of different trauma types in order to find possible exposure biases or other possible effects seen from specific trauma types.
The combined effect of gender and age on PTSD has previously been given little attention in the PTSD literature. However, the results from the present study indicate that it makes sense to consider the combined effect of gender and age on PTSD as it outlines how risk of PTSD or PTSD vulnerability can be seen in a lifetime perspective. The lifespan distribution of PTSD shows that men are most vulnerable to PTSD a decade sooner than women. This difference is of particular interest and needs to be investigated further in future research in order to develop more thorough explanations for the effect.