My mare is a 16yrold thoroubred who is only 15.2hh and jumps over 5 ft but is in shape and loves to jump I couldn't stop her if I tried so it just depends on your horse and if your horse is into it.
I jump my 25 year old horse over very small jumps but we were jumping up to 3 ft until he was about 20 so it depends on your horse, its fitness :)
I completely agree with what Amber said. Take Simply Priceless, he's a 4* , OTTB eventing horse (he's actually at Badminton right now), and he's 16. It really just depends on the horse
It depends on how fit he is. I still jump mine and he's 22
I think it depends on the horse's shape and mental. If he is in top shape and a kind and relaxed horse, I don't see any reason why you shouldn't jump him. However, if he has never jumped in his life, be careful to start with easy exercise and simple demands. Maybe try to see how he reacts with pole work.
Hope it will help you, bye!