Although the core features of mood disorders are essentially the same across a lifetime, traditionally children and older patients are considered somewhat separately because of the special features their phases of life include, and the way these features might influence the overall manifestation of mental disorders and their treatment. Additionally, an early age of onset of any disorder puts forward the question whether this determines a more severe and chronic disease and also poor response to treatment. The ratio of males to females with MDD remains stable across the age spectrum .
It seems that the developmental phase might influence the expression of certain mood symptoms and this is why, for example, pervasive anhedonia or significant psychomotor retardation are rare among depressive children and auditory hallucinations and somatic issues are seen more often in prepubertal children.
The incidence of mood disorders among children and adolescents is reported to have increased during the last few decades. These reports are consistent and they also suggest there is a decrease of the age of onset of mood disorders. The general picture suggests that the prevalence of depression is around 0.3% for preschool children, 0.4% to 3% for school age children and 0.4% to 6.4% for adolescents; the prevalence of bipolar disorder is 0.2% to 0.4% in children and 1% in adolescents. Research suggests that 40% to 70% of children and adolescents with a mood disorder also have at least one comorbid psychiatric disorder. It also seems that childhood depression is prebipolar in the majority of cases . The risk factors as well as the etiopathogenesis for this age group are uncertain.
A worldwide trend is the increase in both the absolute numbers and percentages in the total population of older people. This of course leads to an increase in the number of geriatric psychiatric patients and a shift of the focus of healthcare services. At the same time, geriatric mental patients present with multiple challenges both at the diagnostic as well as the therapeutic level. The prevalence of major depression is estimated to be 2% in the general population over 65 years of age [179–181], with up to 15% having some kind of other mood disorder  and 25% to 40% of patients in the general hospital setting having a subthreshold depression . In residential homes, the accepted value for patients with MDD is approximately 12%, with an additional 30% manifesting a milder form of depressive-like symptomatology [184–189]. The recognition of geriatric mood patients (with a late onset mood disorder) is poor, and less than 50% of hospitalised patients with depression in general medical practice are referred to a psychiatrist, and less than 20% receive adequate treatment .
With regard to suicide and related behaviours, the attempted suicide rate is 1% in children and 1.7% to 5.9% in adolescents, while the completed suicide rate ranges from nearly 0 in children below the age of 10, to a peak of above 18 in 100,000 in boys 15 to 19 years old. The data suggest that among 15 to 19 year olds, the suicide rates have quadrupled over the last four decades, and the reason for this is not known. Unfortunately, suicide is currently the fourth leading cause of death in children aged 10 to 15 years and the third leading cause of death among adolescents and young adults aged 15 to 25 years. The suicide method is the most significant factor in determining whether the attempt will result in death. The great majority of attempts among children and adolescents have little lethal potential, partially because of restricted access to lethal material and inadequate cognitive potential to plan a successful attempt. What is unique in this age group is suicide imitation and contagion. This means that the suicidal behaviour increases in adolescents following exposure to well publicised news stories of suicide or a film involving a teen suicide, but this seems to concern vulnerable individuals and not the age group as a whole [191–193]. At the same time, geriatric patients with depression have up to 1.5 to 3 times higher morbidity , with the lifetime risk of suicide being as high as 15%; almost 10% of them die annually .
The etiopathogenesis of mood disorders in children and adolescents is not well understood. It is an age group that combines developmental vulnerability and high potency for neuroplasticity and compensation for any insults. It is generally believed that genetic factors play a significant role, however there are vague data in support of this and no clear conclusions can be made. Non-shared environmental factors might also play an important role . At the cognitive level, the theoretical approach suggests the presence of cognitive distortions similar to those seen in adults, but again data are inconsistent and scarce.
Traditionally there has been significant interest on the family interactions and their relationship to the development of depression, but the conditions are usually complicated and difficult to interpret. The most difficult problem is that when the family environment is problematic, there is a high probability of a genetic vulnerability in the family and sometimes in both parents. However this does not exclude the possibility that the environment can induce a kind of emotional vulnerability in the child by shaping the early experiences. Depressed parents may model negative cognitive styles and poor self-esteem, leading to a deficit of social problem-solving skills and ability in coping with stressful life events; marital conflict and lack of an adequate family support system, especially when a mental illness of the parent(s) of an early onset is recurrent, and disruption of parental functioning, put the child at a high risk for any mental disorder but especially for a mood disorder. In this sense, it is understandable why family conflict is the most frequent event adolescents report they have experienced before they manifest suicidal behaviour. There are several studies suggesting that depressed children and adolescents might experience more stressful life events such as interpersonal losses, problems in relationships, parental divorce, bereavement, physical abuse and suicide in the environment [197–200].
The conclusion concerning the etiopathogenesis of mood disorders in children and adolescents is that genetics clearly plays at least a moderate role, while both shared and non-shared environmental influences appear to also be important.
However, late onset mood patients are less likely to have a positive family history for mood disorders compared to younger patients [201, 202] and are more likely to manifest structural changes of the CNS [177, 203, 204]. Neuroimaging studies have reported a variety of morphological disturbances, which clearly differentiate late-life depression from depression of younger ages [204–210], suggesting an association to an increased severity of subcortical vascular disease and greater impairment of cognitive performance . Moreover, major depression is more common and more severe in patients with vascular dementia .
Clinically, depression in children and adolescents presents with the same core features manifested in adults. Some minor differences suggest the presence of irritable rather than depressed mood, and failure to attain expected weight gain instead of weight loss. Among preschool children, lack of smiling, apathy towards play, lack of involvement in all activities, physical issues, and physical aggression are common signs, while among school-aged children, deteriorating school performance, increased irritability, fighting, or argumentativeness and avoidance of peers may signal depression. Exacerbation of anxiety symptoms and refusal to attend school are not uncommon among children who are depressed. Switching from unipolar depression to bipolar disorder is significantly higher in children than it is in adults, and it reaches 32% within a 5-year period. Also, it is reported that in children, mania might present with a chronic instead of an episodic pattern, with mixed and rapid cycling features instead of classic manifestations and highly comorbid mental disorders. These suggest that childhood-onset bipolar disorder is a more severe form of the illness, and relatively treatment resistant. The main disorders that should be differentially diagnosed are attention deficit hyperactivity disorder (ADHD) and disruptive behaviour disorders .
Various studies of depression in older people have reported that mood is more often irritable than depressive , and also several symptoms such as loss of weight, feelings of guilt, suicidal ideation, melancholic features, hypochondriasis as well as associated symptoms of psychosis can be more frequent [215–219]. However, these findings vary across studies. Many of these patients manifest a type of behaviour that can be characterised as 'passive-aggressive' or 'self-aggressive'. They refuse to get up from bed, eat, wash themselves, or talk. Also, they often hide important information concerning severe somatic disease and in this way they let it go untreated.
Somatic symptoms are difficult to assess and, as a general rule, doctors should avoid assigning this symptomatology to an underlying mental disorder. It is highly likely the patient indeed has a true 'somatic' disorder even in cases where the doctor is unable to diagnose it . However, it is clear that older depressives manifest more somatoform symptomatology, in comparison to younger depressives. In this sense, the concept of Masked Depression  used to be popular in the past, but today it is not accepted by either classification system although it is accepted that the onset of health concerns in old age is more likely to be either realistic or to reflect a mood disorder . Percentages of comorbidity between depression and physical illness vary from 6% to 45% [222, 223]. The large discrepancy reflects the difficulty in the application of operationalised criteria for the diagnosis of depression in patients with general health problems. Greater overall severity of medical illness, cognitive impairment, physical disability and symptoms of pain or other somatic issues seem to be a more important predictors of depression than specific medical diagnoses .
About 38% to 58%  of older people with major depression also fulfil criteria for an anxiety disorder, while many authors have suggested that the presence of anxiety in older people should be considered as a sign of depression, even in cases, which lack true depressive symptomatology .
In older individuals there is an increased possibility of the coexistence of depression and dementia, or some other type of 'organic' decline of cognitive disorder. The syndrome of 'pseudodementia' has also been described . This term refers to the manifestation of dementia symptomatology, which in fact is due to depression and disappears after antidepressant therapy. The emergence of late onset bipolarity in the sense of an ongoing dementing pathology has also been described [107, 139, 140].
Suicide constitutes an important health problem for older people. Older men are at a higher risk for completing suicide than older women. The coexistence of a serious somatic disease, such as renal failure or cancer, represents a major risk factor for a well planned suicide attempt . Other risk factors include loneliness and social isolation, usually as a consequence of bereavement. Failure to follow medical advice in serious general medical conditions could be considered to be a form of 'passive suicide'. However, 'rational' suicide plans are not common even in severely ill patients. There is a possibility of acute-onset suicidal plans (after an acute incidence concerning general health, for example, stroke or heart attack) .
The psychological treatment of children and adolescents with mood disorders are similar to those for adults. However, there is a significant controversy concerning pharmacotherapy. Double-blind studies are missing and it seems that these age groups are particularly vulnerable for the induction of suicidality by antidepressants. Flouxetine, quetiapine and lithium are the better-studied agents in terms of efficacy in these age groups [230–240]. Electroconvulsive therapy (ECT) and transcranial magnetic stimulation (TMS) might be reasonable alternatives if initial therapeutic attempts fail .
The pharmacotherapy of late-onset mood disorder includes the cautious use of antidepressants including amitriptyline, imipramine, nortriptyline and the SSRIs, which are the most widely prescribed antidepressants among the geriatric population because of their favourable side-effect profile, relative safety in overdose, ease of use and smaller dosage adjustment, making them first-line choices. Also, venlafaxine, mirtazapine, and bupropion can be useful.
For bipolar cases, lithium and anticonvulsants are useful, although they are not well studied in older patients . They are mostly used in cases of refractory depression for the augmentation of antidepressant therapy. Antipsychotics, especially second generation ones, could be used although there is a warning for a higher mortality because of their use in older patients. ECT is another option with many studies reporting better outcomes in older than in younger patients. However by far the most troubling side effect of ECT, especially in older people, is cognitive impairment.
Psychotherapy is also an option [243, 244]. The presence and severity of medical illnesses, physical disability, cognitive impairment and psychomotor retardation make psychotherapeutic intervention difficult and affect its efficacy and success. The form of psychotherapy should be adjusted to the patient's personality, behaviour patterns as well as his/her cultural and educational level. Behavioural therapy, cognitive behavioural therapy and problem-solving therapy have been extensively studied for their effectiveness in the treatment of depression in older people. Fewer studies have been carried out for the efficacy of interpersonal psychotherapy. Non-standardised psychotherapies, such as psychodynamic psychotherapy and reminiscence therapy, are also proposed as appropriate treatments for geriatric depression.
Combination of pharmacological and psychological treatment is associated with higher improvement rates than pharmacotherapy alone, and considered more effective than either treatment alone in preventing recurrence of depression . In long-term therapies, the addition of psychotherapy promotes adherence to treatment .
Eventually, however, most studies support the opinion that geriatric depression carries a poorer prognosis than depression in younger patients. However, many authors attribute this to factors such as failure to make an early diagnosis and improper or insufficient treatment. For patients with geriatric depression, the prognosis is more dependent on physical handicap or illness and lack of social support, however further research on this issue is needed. Thus, the effective prevention of late-life depression requires attention to maintaining community infrastructure and support.