We typically wait until they're 3 coming 4. They go through so many changes from 2-4 that we wait until 4 so that they've matured into themselves and have a solid sense of body awareness. Any sooner, as we have found, can sometimes lead to future issues.
We had a filly that we lightly started at 2 1/2 and she was a piece of cake. She had the winter and spring off and was started up again in the summer. She had changed so much in her body and mind that the 2nd time around she had a hard time because she was lost in her new self which made what she already thought she knew more challenging. It definitely depends from horse to horse, but we generally wait until 4. They have the rest of their lives ahead of them I like to think its fair to them to allow them to grow up.
That being said, we don't wait until 4 to do basic training such as good ground manners, being ponied on the trail, exposed to stuff here and there, things that make them more functional later on.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.