Juvenile arthritis (JIA) refers to chronic inflammatory arthritis with onset before age 16. Age at onset varies by subtype: some oligoarticular forms can begin in very young children, while certain polyarticular patterns may start later in childhood or during adolescence. This age threshold is part of classification, but age alone does not confirm diagnosis. Clinically, the key indicators are persistent joint swelling, morning stiffness, limping, and reduced range of motion lasting more than six weeks after other causes are excluded. Age at onset also shapes monitoring strategy—growth and development tracking and uveitis screening become especially important in younger children. Early evaluation provides a strong advantage in preventing long-term joint damage and disability.