2 at the earliest. But that depends on the colt, and a 2 year old should definitely not be used heavily. We like to use a 2 year old 2-3 times in that year. Then, as a 3-4 year old he is more mature and ready for heavier use.
Some 2 year olds are very immature, though, and NOT ready for any covering at all.
A good horse is worth more than riches. ~ Spanish Proverb