It SO depends on the horse. There is no right or wrong answer here.

You can start the average horse at two OR three (start is the key word). But at four and five you better be riding the crap out of them or they wont be a good riding horse. Its very difficult to start a horse at four, not impossible, but difficult. Ive started a few seven year olds....much better to start them at two or three.
I was the daughter who wouldn't lift a finger in the house but cycled madly off in the pouring rain to spend all morning mucking out a stable.p067.ezboard.com/bhorseloversdotcom