At the barn I worked at, we had a retired 30 year old pony routinely clearing the pasture fences. Her boyfriends would bust through the fence, but she just floated over.

If we're getting down to the nitty-gritty, most all human uses for horses will cause at least some discomfort at the end of their lives. Toting around an off-balance beginner for any amount of time will set them off balance.

I think the point is to, personally, choose the best horses for the disciplines. Jumpers should have big, strong, flat knees, for one thing. Take an itty bitty delicate leg, and you're asking for trouble. You can do it, but not a lifetime. Maybe not even five years. But there are horses that can, and have, gone the distance in these sports, and come out of it no less geriatric than their owners/riders.