Is 20 year old horse too old to gallop?

Most of my sister's polo horses are in their late teens and early 20's, and they gallop every day during the polo season without problems. As long as your horse is sound and has no major health issues, he should be fine- but you need to have him checked by a vet before you start any kind of conditioning program with him. Have the vet check his heart, his lungs and so forth. He should also be seen by a dentist to make sure that he's able to chew the feed you'll be giving him. Your vet can advise you on how to prevent soreness and flare ups of things like arthritis. He or she can ( and probably will) give you recommendations on what to feed your horse to help him stay in good shape.