Adult rheumatoid arthritis (RA) most commonly begins in adulthood, often in young to middle-aged adults, although it can start at any age. Childhood-onset inflammatory arthritis is typically categorized under JIA today, and the term “juvenile rheumatoid arthritis” is largely historical. Age of onset matters for diagnostic strategy and differential diagnosis: in children, growth and development, eye screening, and subtype classification play a larger role; in adults, comorbidities, cardiovascular risk, and long-term functional targets are managed differently. However, age alone does not confirm a diagnosis—symptom pattern, clinical examination, and appropriate lab/imaging evaluation remain essential.