What causes young people to get arthritis in tmj?

Bruxism. Chronic grinding of your teeth, bruxism, can contribute to arthritic bone growth can lead to TMJ symptoms. There are several dental appliances and dental surgeries that can help place the lower jae in a more favorable position.
TMJ Arthritis. The most common arthritis cause for young people is trauma or rheumatoid type issues. We do not see primary TMJ rheumatoid very often. We do see trauma to the joint, causing disc displacement and resulting arthritis over several years time in teens mostly. This physical injury is cumulative and manifests as flattening of the condylar head, bone to bone contact with beaking and peaking.
JRA. Juvenile rheumatoid arthritis is one possibility... Not specific to tmj- it can affect any joint, and is an autoimmune disorder of unknown origin. Trauma to the joint from an accident, or even from bite disorders can be contributors to non-arthritic breakdown of the tmjs. Hormone disorders is a disputed possibility of breakdown, and avascular necrosis (avn) can cause severe TMJ changes...
Depends. Th most common form of TMJ arthritis is related to disk problems. But certain arthritides , like rheumatoid arthritis can preferentially affect the TMJ in yound people.