I moved away when my boy was 19, & we were doing 3'9" courses no problem. He's 23 now & is mostly just used as a lower-level school pony, but when I go home for visits, he still has no problem at 3ft. We used to have a school horse whose owner decided to retire him at age 30, but I jumped him 3ft 2 weeks before he left. He loved his job, & I have no doubt that he would have had years left of jumping in him if his owner hadn't decided to move up north & take him with her. It really does just depend on the individual horse's health & quality of life.