Major depressive disorder (MDD) is a prevalent and disabling mental disorder that affects approximately 3.8% of the population, including 5% of adults [1]. It is a growing public health concern owing to its lethal and recurrent nature. Furthermore, MDD is expected to be the leading contributor to the global disease burden by 2030 [2]. Therefore, it is important to accurately identify the status of MDD in the general population, such as the prevalence and organization of symptoms, in order to distribute health resources and develop appropriate screening and treatment.

The prevalence of MDD varies by age [3], but it is not necessarily true that it increases with age. Some studies have found that depression is more common in younger age groups, particularly in adolescents and young adults [4]. However, MDD can occur at any age, and older adults are not immune to it. In practice, MDD is more likely to be underdiagnosed and undertreated in older adults [5], possibly due in part to stigma, physical health problems, and other factors that can complicate diagnosis and treatment [6]. In other words, MDD symptoms might resemble characteristics inherent to the aging process, making it difficult to recognize MDD in older adults [6]. Thus, gaining insight into differences in the age-specific symptom profile of MDD may contribute to the improvement of diagnosis and treatment.

It is known that the presentation of depressive symptoms differs according to age [7]. Psychomotor agitation, hypochondria, and somatic symptoms were more common in older adults, whereas guilt, which can be perceived as a cognitive symptom, was more common in young adults. However, most of the previous studies were conducted only on several depressive symptoms using the Hamilton Depression Rating Scale [8], which does not include all Diagnostic and Statistical Manual of Mental Disorders (DSM) depressive symptoms. Regarding age, people were usually divided into two groups, and middle-aged people were inconsistently grouped. Furthermore, studies on the difference in depressive symptom profile in Asian populations are scarce.

As sociocultural factors also influence the manifestation of symptoms of depression and mental habits [9], it is essential to study the epidemiology of MDD across countries with different sociocultural backgrounds and establish countermeasures, including optimized mental health policies. Thus, this study aimed to investigate the 1-year prevalence of MDD and the presence of a range of depressive symptoms across different age groups within the Korean population using data from a nationally representative sample from Korean Epidemiologic Catchment Area (KECA) studies.

Using data from a nationally representative sample, we examined the 1-year prevalence of MDD and depressive symptom profile by age group. The main findings of this study were as follows: 1) the 1-year prevalence of MDD was stable across all age groups. 2) However, 13 symptoms (50%) differed depending on the age group. That is, the older age group was more likely to be associated with melancholic symptoms, including depressed mood, insomnia, awakening 2 h earlier, psychomotor agitation, and cognitive symptoms, including suiciderelated symptoms, along with feeling guilty. Whereas, the younger age group was more likely to be associated with atypical symptoms such as increased appetite, weight gain, hypersomnia, and fatigue.

Our results are inconsistent with previous findings that the prevalence of MDD varies by age [3]. Previous studies have suggested that the prevalence of MDD tends to increase with age, peaking in adulthood [4]. However, it is essential to recognize that prevalence may vary depending on factors such as region, culture, and data collection methodology. Nevertheless, regardless of age, the prevalence observed in our study was lower than the global prevalence [11,12]. Sociocultural influences on the concept of depression may contribute to the low prevalence of depression in some cultures [13]. The suggestion that non-Western people have lower rates of depression is interpreted to reflect denial of the illness or a tendency to express depression somatically [14]. Due to these patterns, the interpretation of this alternative presentation may influence epidemiological estimates [15].

This study found differences in depressive symptom profile by age, similar to previous studies [7,16]. The older age group exhibited associations with melancholic symptoms, particularly somatic symptoms, which were the core symptoms of melancholic symptoms, such as psychomotor agitation, insomnia, and awakening 2 h earlier. Furthermore, for insomnia and awakening 2 h earlier, the value of OR increased with age. These somatic symptoms could be attributed to aging or the presence of chronic somatic diseases in older adults. The manifestation of somatic symptoms at older adults might elucidate why late-life depression has been relatively more often underdiagnosed, as older adults experiencing these symptoms may exhibit attribution bias, thinking that their problems are simply consequences of growing older and they may not feel the need to seek help [6,17]. However, it needs to consider that physiological changes in older adults, such as an advanced sleep phase, could contribute to these symptoms. For example, awakening 2 h earlier could be a manifestation of an advanced sleep phase, potentially leading to a misperception of insomnia. Therefore, further research into these findings may be necessary.

Moreover, cultural factors can play an important role in the experience, presentation, diagnosis, and treatment of depressive symptoms [18]. Although depression has been observed across cultures, cross-cultural presentations of depressive symptoms may vary [19]. Particularly, somatic symptoms of depression may be more heavily emphasized among non-Western populations, resulting in higher rates of somatization compared with Western populations [20]. It might be worthwhile to ask persons who report physical complaints about their mood, as older adults might not be used to expressing mood problems and might focus on physical health [6]. Furthermore, in relation to the stigma attached to depression among the Korean elderly, future research needs to develop a depression scale that fits the emotional and cultural characteristics of the Korean elderly, rather than simply applying an international scales.

Associations with the older age group were also observed in cognitive symptoms, such as feeling guilty, thoughts of death, and thoughts of suicide. In particular, contrary to previous findings [16], suicide-related symptoms (i.e., thoughts of death and thoughts of suicide) exhibited an association with the 60 years or older group. According to the interpersonal theory of suicide, suicide-related symptoms are the result of feelings of thwarted belongingness and perceived burdensomeness. Older adults experience several situations of social stress and adverse social experiences, such as retirement, widowing, or lack of new intimate relationships [21]. In addition to the low sense of belongingness, poor social support, and social integration, the death of close relatives is a highly stressful event and constitutes a risk factor for suicidality. Also, for older adults, physical illnesses (e.g., heart disease, stroke, and chronic obstructive pulmonary disease) could represent an acute stressful life event and have been associated with a higher risk of subsequent depression, which could be associated with suicide; some physical conditions may cause chronic pain, a well-known risk factor for suicide [22]. Finally, functional impairment could play a further consistent role.

This study found the younger age group to be associated with atypical depressive symptoms, similar to previous studies [23]. Atypical depression is characterized by higher levels of inflammatory and metabolic dysregulation, reduced serotonin synthesis, and hypoactivity of the hypothalamus—pituitary—adrenal axis [24]. Increased appetite and weight gain associated with the younger age group may be attributed to the slowing down of life because of loss of interest and emotional eating, as well as elevated levels of cortisol and inflammation in this context [25]. In particular, hypersomnia, psychomotor retardation, and worse in the morning remained statistically significant after adjusting for gender, marital status, years of education, and employment status. Furthermore, fatigue and decreased libido were newly associated with the younger age group. It has been reported that MDD with hypersomnia is common in younger age groups and that there are many complaints of symptoms related to fatigue (loss of energy) [26], which is relatively consistent with the differences in depressive symptoms according to age in this study. Regarding decreased libido, one study found that it was commonly observed in older adults with depression [27], whereas other studies found that younger adults with depression were more likely to have decreased libido than older adults with depression [16,28]. In our study, we observed an association between decreased libido and the younger age group. Consistent with the previous results, not only a decrease in sexual desire and sexual function with aging itself but also the lack of a living partner might explain our finding that older patients had lesser sexual dysfunction due to depression than younger ones [7]. Nonetheless, it remains unclear how sexual interest is differently affected by the pathophysiologic process of depression by age group. Nonetheless, it is recommended to consider sexual dysfunction, particularly in the treatment of younger individuals with MDD. Additionally, individuals with atypical depression often experience a more chronic, unrelenting course of depression than those with typical depression and could be associated with increased comorbidity of other psychiatric disorders and greater functional impairment [29]. Therefore, in terms of long-term outcome estimation, these characteristics should be taken into account in younger individuals with depression who are highly associated with atypical depressive symptoms.

When interpreting the results of this study, its limitations need to be considered. First, this was a cross-sectional study in which existence of its individual depressive symptoms and diagnosis of MDD were relied on the respondents’ retrospective reports. To minimize recall bias, we assessed and analyzed individual depressive symptoms and MDD diagnosis within the past year. Second, although our study demonstrated an association between age groups and depressive symptoms, the causal association between the two remains unknown. Accordingly, it is necessary to conduct longitudinal studies to confirm accurate temporal relationships. Third, we could not thoroughly analyze somatic symptoms associated with depression as the K-CIDI diagnosis of MDD was based on the DSM-IV and ICD-10 definitions, which do not encompass common somatic symptoms such as gastrointestinal trouble, headache, and frequent palpitation. Finally, the categorization of age groups into three cohorts—specifically, older adults as 40-59 years and 60 years or older—necessitates caution in interpreting the results. Future research that treats age as a continuous variable is needed to provide more nuanced insights. Furthermore, we need to study the severity of symptoms, chronicity, number of episodes, and onset of the disease. Nevertheless, this study has the following strengths: for the first time, the depressive symptom profile by age group was investigated in the general adult population of Korea. Furthermore, a validated structured interview tool, K-CIDI, was employed to assess MDD.

In conclusion, the depressive symptom profile of MDD in the general Korean population varies by age group. In particular, the older age group was more associated with somatic symptoms and cognitive symptoms, including suicide-related symptoms. These differences should be considered owing to their potential relevance to treatment response and prognosis in the clinical setting. Moreover, this study may provide significant information for public policymakers to establish national mental health policies to reduce the social burden of MDD, especially in developing appropriate screening and treatment by age group.