Ankylosing spondylitis (AS) has 2 main modes of onset: juvenile-onset AS (JoAS) and adult-onset AS (AoAS). It is not known whether JoAS is a subtype of AS, or AS modulated by early age of onset and longer disease duration. We performed a systematic review of the literature, identifying 12 articles and 1 abstract directly comparing JoAS and AoAS cohorts, with observational study design. Patients with JoAS appear to have more peripheral joint involvement both clinically and radiographically (especially knees and ankles) and more root joint involvement (hips and shoulders); they are more likely to proceed to hip arthroplasty and often initially present with peripheral rather than axial symptoms. Patients with AoAS appear to have more axial symptoms and radiographic disease, particularly in the lumbar spine, and worse axial metrology. In terms of other characteristics, more evidence is needed to confidently state whether JoAS and AoAS are different.