**Core Concept:**
Juvenile Rheumatoid Arthritis (JRA) is a chronic inflammatory disorder characterized by persistent joint pain and swelling in children under the age of 16. The disease primarily affects the joints and can lead to joint deformity, disability, and systemic complications. Age criteria are essential to differentiate JRA from other childhood arthritis conditions.

**Why the Correct Answer is Right:**
The correct answer is A: **Juvenile Rheumatoid Arthritis (JRA) is defined as arthritis in children under the age of 16.** This age criterion is crucial for distinguishing JRA from other childhood arthritis disorders. In addition, the disease typically starts within the first six months of life, and the symptoms often persist for years.

**Why Each Wrong Option is Incorrect:**

Option B (JRA occurs in adults) is incorrect because JRA exclusively affects children under the age of 16. JRA is distinct from adult-onset arthritis, which typically occurs in individuals aged 16 and above.

Option C (JRA is caused by infection) is incorrect because JRA is a non-infectious inflammatory disorder. The underlying cause of JRA remains unknown, and it is considered a complex autoimmune disease with genetic and environmental factors contributing to its development.

Option D (JRA is characterized by joint pain only) is incorrect because JRA is a multi-systemic disease that can affect various joints, causing not only pain but also swelling, stiffness, and limited mobility. Additionally, JRA can lead to systemic complications such as heart, lung, and skin involvement.

**Clinical Pearl:**

Juvenile Rheumatoid Arthritis (JRA) is a complex autoimmune disorder affecting children under 16 years old. Its age criteria enable the differentiation from other childhood arthritis conditions and provide insight into the disease's unique features, including its genetic and environmental factors, multi-systemic nature, and the potential for systemic complications.