If your horse is telling you he's enjoying it, there's no reason to stop! Like the others have said, he'll let you know when it starts to become a problem.
Keep it light and keep it fun, just to protect him since his body is getting older. No reason to do anything too crazy or take huge jumps or really hard courses. He may be enjoying jumping, but his body is getting older so you have to be more careful with it.
It's great that he's in such good shape at his age and still going so strong. I would think that riding and even jumping him as long as he can handle it will keep him going even longer.