The horse I jump is a 20 year old, sound, Canadian Thoroughbred. As long as they enjoy it and it isn't detramental to their health then it is fine. We are jumping up to 3 foot and he loves it, doesn't even phase him.
Yeah one of the ladies that shows in my division rides a 24 year old gelding. He's sound and healthy and happy as can be. On the other hand, I have seen people have to retire their 8 year olds because are so broken down already that they can't do anything any more. Which is very sad and infuriates me.
My horse is over 20 & he can still do first field fox hunting for over 4 hrs =] it just depends on how hard they are jumped, their conformation, & how well you take care of their legs...also other injuries etc
We have a pony that is very, very old. Not sure how old exactly, his owner claims he is 40+ but I sincerely doubt it, as she is only 7 and he really doesn't act that old or seem/look that old. Anyhow he jumps up to 4'0 or so and he loves it. (:
There's another that is 35+ and he has horrid feet but, he loves to work and is jumped a tiny bit and doesn't act like he's in pain.
If the horse/pony ect enjoys what they do then fine keep jumping them and do what your doing but as soon as they start turning it in and they have had enough try a few more times still not working cos there old, I would give it a break. I jumped my 13.2hh untill it was 25. It was sound and loved what it did.