Juvenile arthritis is arthritis in anyone under the age of 16. Adults may experience several types of arthritis such as osteoarthritis, a condition in which your bones and joints wear down over time, or rheumatoid arthritis, an autoimmune condition in which your immune system is overactive and ends up attacking joints.  There are multiple types of juvenile arthritis, but, like rheumatoid arthritis, they all involve an overactive immune system, explains Jay Mehta, MD, a pediatric rheumatologist at the Children’s Hospital of Philadelphia.