Definitely way late to this thread and I have not read anything but the first page so bear with me.
Anywho I will start colts are two. I mean sure if it's a little before that and I start getting them used to the saddle my 110lb self laying (not riding, laying) on them isn't going to hurt them. At two years old they start work, knowing what legs are, flexing and bending, etc. There is SO MUCH you can di with a two year old without even getting out of a slow trot. I don't work two year olds hard. I generally do all the leg work, walk/trot and a little brief canters. That way they are getting experience and geting in shape too. Then we just progress as the horse does at the colts pace.
Now I would not start an Arab, warmblood, or anything along those lines that young with work, but my paints and QHs are a totally differant breed. They are sound enough to learn at that age. I don't expect them to lope for hours or slide 30 feet, nor do I expect them to run the barrel pattern or jump... Just know how to move off my leg, frame up when I ask, and have a solid foundation. That does not take as big a toll as people think. The majority of the time we are at a walk learning these things. Posted via Mobile Device