I have a friend who retired her 28 yr old half arabian mare from her lesson program, only because she was getting cranky, lol. She was still very sound and healthy, still kicking at 30 now, she was just tired of toting kids around the arena
Just pay attention to your horse. If you notice stiffness, joints creaking/cracking, reluctance in being caught and/or saddled, then you should have your vet out to check for arthritis or other possible causes. Working an arthritic horse is usually good for them, you just have to have a longer/slower warm-up and cool down, and follow your vet's advise on supplements, medications, or injections. Stall time is bad for arthritic horses. The more time standing still, the stiffer the joints get.