The World Mental Health Survey Initiative 2000 conducted CIDI surveys between 2001 and 2005 in different countries of which 17 study findings have been published . The study cohorts belonged to the age groups of 18 years and above. Lebanon and Israel were the only two countries which reported from the Middle East. The present study in the same time period, to our knowledge, was the first CIDI survey among the adolescents and youths between 14 and 23 years in this region. This cohort closely resembles the New Zealand study conducted during a similar time period which included those aged 16+ years though only the 18+ age group was analyzed.
The use of WHO-CIDI is justified as it is the only available instrument based on extensive cross-national field trials. The adult CIDI surveys have consistently shown that anxiety disorders have a median age of onset in the early to late teens, while mood disorders have a median age of onset in the early to mid twenties . The 18+ age group respondents in this study contributed only 16.70% to the overall sample but still the results of adult studies have been used for discussion in the present paper, in spite of inherent limitations, so as to help corroborate the findings in the preceding statement and also facilitate international comparisons based on one standardized instrument. Hence the findings of other studies which have used different instruments on the respondent sample similar to ours have been avoided in discussion as far as possible.
The response rate in the present study was 91.61%. In the literature, it ranged between a minimum 45.9% in France and a maximum 87.7% in Columbia while the respective response rates were 70% for Lebanon, Israel 72.6%, and New Zealand 73.3% . The higher compliance in the present study may be due to the fact that the study was conducted with the constraints and advantages of a school setting. It is possible that the response rate from community-based surveys rather than school-based ones would strongly hinge on the adults' choice to either participate in the study or not. Therefore, it appears that a community survey tends to have a lower response rate.
The initial finding of a prevalence of 17% of depressive symptoms based on self reported CDI  in the source sample translated during the same time period to markedly lower estimates of lifetime prevalence of MDD and BMD. Studies in adolescents and youth using different research methodologies have reported variable estimates of depressive symptoms from about 9% to between 25% and 40% [23, 24]. The present study suggests that one need not be alarmed by results based on depressive symptoms only but it endorses the adult studies findings that mental disorders do begin earlier in life. A vast majority of adults with serious mental disorders experience a combination of panic, generalized anxiety, depression, phobia and substance abuse which differ substantially in their ages of onset. Anxiety, oppositional-defiant and attention-deficit problems typically have earlier ages of onset. It is hypothesized that the cumulative effect of these disorders could be of causal significance and hence measures need be taken to reduce the prevalence of serious mental disorders in adolescents and youths .
In the prevalence estimates over previous 12 month period, lower prevalence is noted for MDD, specific phobia, ASA and any anxiety and any mood category as compared to the lifetime prevalence suggestive of an earlier onset. The decrease could perhaps be attributed to natural causes or available treatment. The linear increase seen in the prevalence of any mood disorders in the lifetime estimates confirms the earlier findings that mood disorders have a later age of onset .
The age of onset distributions for lifetime estimates overlap with the findings of adult studies from other countries  necessitating the targeting of this population in Oman for further investigation and intervention if necessary. The results corroborate with other studies which show that impulse control disorders have the earliest age of onset distributions, an early median age of onset and a narrow age of onset risk between 13 and 21 years. The estimation was not done for the other subgroups, as the respondents were fewer than 30 in the study sample. The median age of onset for specific phobia was 13 years and well within the 7-14 years range reported. But the IQR of 7-22 years in this study varied from the narrow IQR of 8-11 years in other studies. This could be due to a relatively smaller sample size in a limited age range. The other anxiety subgroups had a later median age of onset (median 25-50 years, IQR 31-41) in adult surveys. This study showed a similar trend, with a later median age of onset at 18 years (IQR 8-22 years) for the any anxiety category when compared to specific phobias. The difference between the impulse control group and phobias in comparison with the other anxiety disorder groups can be attributed to wider cross-national variations in the latter. This must be interpreted with caution due to the methodological considerations . The discrepancy in IQR for specific phobia in this study compared to others could be due to the same reason. For mood disorders, the reported prevalence is consistently low until the early teens, at which time a roughly linear increase begins that continues through the late middle age, with a more gradual increase thereafter and this study results are on similar lines.
Studies seem to indicate that the odds ratios for anxiety and mood disorders are higher in the recent cohorts compared to the older cohorts . These studies compared each cohort of approximately 15 years and consisted of 4 cohorts ranging from 18 years to 65+. This study being restricted to three narrow cohorts of 14-16, 17-18 and 19-23 years perhaps did not exhibit the above trend.
The female gender proved to be a strong predictor of lifetime risk of MDD, AMD and specific phobia. A differential willingness hypothesis has been proposed as a plausible explanation of the observed finding that women report higher rates of anxiety and depression than males who tend to under report, thus leading to biased estimates . Alternatively, there may be gender-specific factors that contribute to such a discrepancy, but it is beyond the scope of discussion in this paper.
This study demonstrated a significant increase in lifetime prevalence in agoraphobia without panic and SAD/ASA in the 17-18 year age group compared to the lower and higher age groups. Cross-nationally these disorders show an inverted U-shaped trend and tend to decrease as age increases . The estimates of mild and moderate severity in the anxiety spectrum in this study appear to contradict the above findings due to this study being restricted to three narrow cohorts in a restrictive age range. Also, as the anxiety spectrum illnesses tend to be characterized by somatic distress, it is possible that cultural factors may have played a part in the present trend. Further exploration into this phenomenon is therefore warranted. The rates for lifetime prevalence in the three classes of having any disorders are comparable to other cross national studies  which suggest the IQR of 9.9-16.7% for any anxiety disorders and the IQR of 3.3-21.4% for any mood disorders. The younger cohort in our study could account for lower bound estimates of prevalence rates. The Impulse control disorders are comparably least prevalent (IQR 3.1-5.7%) across countries and our sample showed a similar trend for lifetime prevalence. The narrow age groups of the respondents in our study accounts for a limited age range of onset risk in the mood disorders group.
The WMH measures of severity were applied only to 12 month cases as there is at present no way to estimate the severity of lifetime cases. The prevalence of severity is quite similar to the available findings in other countries where the majority of cases between 33% and 90% (IQR 40-53%) were rated mild , but even mild cases could be impairing and evolve into more serious disorders over time . The odds of severity were 80% less in females which can be attributed to their willingness to discuss personal problems which had a cathartic effect and hence reduced the severity of the problem. It has been noted that individuals who air their emotional experiences are likely to have positive health outcomes . The no difference in the odds of severity across age groups was because of the narrow age range in this study.
A recent review of the magnitude of mental disorders in children and adolescents from recent community surveys across the world demonstrated that though there is substantial variation in the results depending upon the methodological characteristics of the studies, the findings demonstrate that approximately one fourth of youths experienced a mental disorder during the past year, and about one third across their lifetimes. Anxiety disorders are the most frequent conditions in children, followed by behavior disorders and mood disorders . A similar trend was noted in this study. Belfer, reporting the findings of different researchers', states that children with depression, ADHD and conduct disorder have higher rates of health care utilization, impose costs on society in terms of education, and are a burden on the criminal justice system and on social services . In 2002, only 7% of the countries worldwide (14 out of 191) had a clearly articulated specific child and adolescent mental health policy . Ironically, the countries with the highest proportion of children and adolescents in their populations are those countries that are most likely to lag behind in child and adolescent mental health policy .