We have several school horses that are that age jumping, showing, and sound. I wouldn't train a horse to jump at that age but if they've been jumping for a while I wouldn't worry about it. Just moniter them carefully for any signs of pain. Sometimes it won't show itself as lameness but at some point they start getting more short strided (always taking the chip in instead of the long distance), miss their lead changes, even start stopping at fences. That's when they're retired to flat lessons.