It really depends on the individual horse. All horse mature at different paces. Horses who tend to be on the bigger side (like lets say 16.2-17hh+) are usually slower matures. People seem to think JUST because the horse is big, automatically means that the horse is mature enough for somebody to ride it, which is false.
I have my 2 year old (shes going to be 3 on the 21st of January) who I started a while ago when she was 2 1/2.
Its one thing to get on them and do small basics, like this is how you turn, etc. and doing minimal circles and turns as possible.
As long as you know what you are doing and keep it short you shouldn't have any trouble.
I should add that the reason I can start my horse at this age is because 1. She's definitely mentally mature enough, she has the behavior of a 21 year old. I kid you not. 2. She is a very strong horse, we've had a vet check her and he thought that she could start light training about now.
Always talk to a vet about this stuff too, it doesn't hurt.