The aim of this study was to determine age at onset of psoriasis in a population-based twin sample. Questionnaire-data on age at onset of psoriasis in 10,725 twin pairs, 20-71 years of age, from the Danish Twin Registry, was collected. The association between the ages at onset of psoriasis was calculated using survival regression analysis. Median age at onset was 25 and 28 years among women and men, respectively. The correlation between the ages at onset of psoriasis was 0.84 (bootstrap standard error = 0.044) in monozygotic twin pairs and 0.60 (0.051) in dizygotic twin pairs, permutation p = 0.001. Age at onset of psoriasis in the index twin did not predict risk of psoriasis in the co-twin, hazard ratio (per year of later onset) = 1.01 (0.99-1.03), p = 0.434. In conclusion, these data support that age at onset of psoriasis is, in part, genetically determined. The results do not support that early-onset psoriasis is more genetically determined.