**3. Epidemiology**

People with OCD seek medical help when their daily functionality is seriously compromised or they experience severe anxiety, and it has been reported that these individuals start seeking professional help after an average of 7 years from the onset of these symptoms. It was found that adolescents with OCD often hide their symptoms and delay seeking help due to several reasons such as inability to recognize their symptoms as disease manifestations, embarrassment, fear of being stigmatized by other people, and believing that what they experience is transient [29, 30]. Furthermore, because adolescents hide their symptoms, it is difficult to determine the actual prevalence of the disorder, and when they seek professional help, they may be misdiagnosed as depression or anxiety disorder due to not mentioning their symptoms [31].

In the past few decades, knowledge of OCD has increased, but studies were mostly done in adult population and less studied in children. Although the first study about the prevalence of OCD in children was reported in 1970, there are few population-based studies presented about the prevalence of OCD in children and adolescents recently [32]. The prevalence of OCD in children and adolescents has been reported between 0.5 and 3% [33, 34]. In a recent study, in 16 European countries, median prevalence of OCD was found 0.7% [35].

It is predicted that OCD is the fourth frequent psychiatric disorder after phobies, substance use disorder, and depression. Studies in different countries and cultures show that OCD prevalence is independent from cultures [27]. Previous epidemiological and clinical studies show that OCD is more frequent among males prior to adolescence and during childhood, the difference between the sexes diminishes to a similar rate as the age advances, and the prevalence rate does not differ between sexes during adolescence and adulthood, and the rates are equal in both sexes at this time [36–46]. Although it was reported in the literature as early as 2 years of age, OCD usually begins at late childhood and early adolescence in youth. Age at onset of the OCD is averagely 10 years old, but age of diagnosis is around 13 years old [47]. Childhoodonset OCD's onset age is approximately 8–11 years old in boys and 11–13 in girls [48].

OCD has adverse effects on family, school, and social lives of children and adolescents [49, 50]. The age of onset has significance in terms of the disease progression. Several studies have detected that OCD often starts at late adolescence and early adulthood period [51, 52]. Studies with adolescents showed that OCD development risk is higher at late adolescence than early adolescence [53]. It is very important to detect OCD at its early stage, because studies indicate that 50% of the adult patients develop the disease during childhood or adolescence [47, 54–56].
