There really is no age limit, every horse if different. I believe the horse will let you know when they are no longer interested, able or willing to jump. We just retired (from showing) the schoolmaster I spent my first year on, and he was 22 doing the 2'9"s. By the end of the season we could tell that he was getting tired and was ready to retire after a very sucsessful career. He still jumps at home once and a while - he much prefers that over flatwork anyday!
I know someone else who showed their horse up to the age of 25...they even came in champion at their very last show! So really it all depends on your horse.