Abstract Objectives To investigate whether the clinical expression and disease outcome in psoriatic spondylitis (PsS) may vary according to age at disease onset. Methods This study included 70 patients from a unique outpatient spondylitis clinic followed on a regular basis with a standard protocol. Patients were diagnosed with PsS according to ESSG criteria plus radiographic sacroiliitis (SI). Outcome parameters included: disease activity, functional evaluation, radiological damage, mobility restriction, and enthesitis score. Patients were divided into those with disease onset before 40 years (young-onset PsS) and those with onset over this age (late-onset PsS). Clinical features and outcome parameters were compared between groups. Results There were 44 men and 26 women. Thirty-nine (M:F ratio 1.8) patients had disease onset before 40 years and 31 (M:F ratio 1.6) over this age. HLA-B27 correlated with PsS susceptibility (34% vs 7%, RR 6.4, p < 0.0004), but it was found over-represented in young-onset PsS compared to late-onset cases (51% vs 13%, p = 0.001). Young-onset cases tended to have a higher frequency of family history (26% vs 13%), bilateral SI (62% vs 29%, p = 0.013), isolated axial pattern (31% vs 13%), and enthesitis (54% vs 29%, p = 0.09). In late-onset PsS there was a higher frequency of unilateral SI (71% vs 38%, p = 0.013), polyarthritis (45% vs 23%, p = 0.022), and silent axial disease (32% vs 10%, p = 0.022). Outcome parameters were similar between groups. Conclusions Clinical picture but not outcome parameters, varies according to age at disease onset in PsS. The correlation between HLA-B27 and young-onset PsS supports the notion that disease susceptibility and disease expression are not driven by the same gene in this entity.