My breaker [an unraced TB] is two. I see nothing wrong with it, done right, and depending on the individual horse's physical and mental maturity [mine is more than ready to come into work at two years 4 1/2 months]. I recall reading somewhere that horses shouldn't be in more days of work a week than they are years old, so once or twice a week is ok for a two year old, up to 3 times per week for a three year old, and so on and so forth, until they come into full work [6 days/week] age six.
Young horses don't have very long attention spans, either, so shorter rides are better. My breaker doesn't get schooled for longer than 20 minutes at a time. She gets schooled, then trail ridden, then schooled again briefly so she doesn't get barn sour... presently three days a week or so, but once she's more established I'll drop her back to once or twice a week tops.