Arthritis is a condition that is usually associated with old age.
But according to NHS England, about 15,000 children and young people in the UK have developed some form of the illness.
Eli, 16, who lives in London, is one of them.
Because it is often seen as something that only affects the elderly, some parents find that getting a diagnosis for their children can be a real struggle.
You can watch the full report from Inside Out London on the iPlayer here.