That is alittle young for true arthritis to develop, but not impossible. I would be concerned there is another issue going on like spinal or disc disease, a neurological issue, or other. I recommend a trip to the vet. We have come a long way with medications for cats...so if it is a back or hip degeneration...at least we can help with pain...if it something else, you can get early intervention.
Dr.V
Dr. D.A. Vamvakias
Veterinarian