If the horse has been taken care of all it's life and has good confo, then they can go strong for ages. Boarder's 36 yo quarter mare who passed away this winter was leading the pack on difficult trail rides up until the week of her death. According to her owner she was never lame once in her life.
Another horse I know is around 16 and completely unrideable.
Lilly will be 10 in a few months and will soon be starting competitve trail and distance riding. Pickles just turned 13! I want to do some dressage with him and both horses will eventually be used in mounted search and rescue volunteer work. I can't see any reason why they wouldn't be able to do this. Some people are prone to passing up horses who are over 15...I say if the animal is healthy and sane, they're missing out.