Genetic variability in the serotoninergic system and age of onset in anorexia nervosa and obsessive-compulsive disorder.

•The age of onset of psychiatric disorders may have etiopathogenic and clinical effects.•The study of genetic pleiotropy in diseases that exhibit significant co-heritability.•Associating two SNPs of the HTR2A with the very-early onset phenotype in AN and OCD.•Common genetic background in AN and OCD affects common aspects of the two diseases such as very-early onset.