I agree with above. I usualy start lunging and ground drivingn at two.
IF they have mentally matured enough, I usually start riding at three. My daughter has decided to leave her fily until four.
If you wait too long, they lose any work ethic. That is why I like to start making them work about two - even if it is only 20 minutes here and there. Then, by the time you are riding, they love to work and are easy to work with.
I got my 2 yr old Arab filly on January 4th, her 3rd birthday was March 15th. I starting lunging her and doing ground work as soon as I got her. She had not been worked with at all. She has been saddled a total of 6 times now, and I leaned over her back for the first time today. She is on a meticulous diet and exercise program. She is a rescue :) Here is a pic: