Wow...this topic is pretty old but I'm gonna reply anyway!
I have a 14.2hh 21 year old QH that I've regularly RACED (not barrels, flat out racing) against a field of other horses numerous times (none were older than

the shortest one was 15hh and the tallest was 17hh ranging from Arabs to TB's back to QH's. My old man cruised on past them all like they were pack donkey's ^^!
Lol it's so funny I showed up on my little old horse and his name is POKEY of all things and when he's just sittin' around he naps hehe, so people were laughin at us. However, I knew that they wouldn't be for very long ^^. Hehe it's so funny because my horse will get about 5 horse lengths ahead then stop accelerating and keep even at that distance. No matter how much they'd encourage their horses he'd always remain that 5 lengths ahead and he still had more left in him. It's amazing how much speed, and more importantly heart, my little old man has.
I'm not racing him anymore due to arthritus in his shoulder and fetlock but I do still ride him a lot (gotta keep him in shape!) so I guess other people will actually have chances now

hehe!