It is unclear whether psoriasis is a progressive disease that requires early aggressive intervention. This population-based study identified patients with psoriasis and psoriatic arthritis (PsA). Survival analysis and Kaplan-Meier life table techniques were used. The study comprised 10,011 psoriasis patients (severe n = 4,618), and 1,269 patients also had PsA. Incidence of PsA increased with duration of cutaneous symptoms (p = 0.0001). Psoriasis diagnosed before age 20 or 30 years, respectively, suggested a lower risk of PsA than psoriasis diagnosed after age 50 years, yet age at first cutaneous symptoms did not predict development of PsA. No clear association with disease severity was found. PsA incidence appeared stable with longer duration of psoriasis, but further data are needed to firmly establish the relationship with age of psoriasis onset.