On the bigger bodied guys, I'd prefer to wait until they're about 5 years old. Not sure on Gypsies, but on Clydes and the like, that's when they actually stop maturing. You'd really want your horse to be fully developed before doing extensive riding. Now if you just want to hop up on a 3yo that has had extensive ground work and maybe walk around the pasture once a month, I'm sure a 2000lb horse isn't going to be bothered by the extra 150lb rider. But the general rule I go by is not riding before the horse is fully developed, which just takes longer on the beasties.