The average horse's growth stops at five- some with a shorter or longer time period depending on the breed. Personally, I wouldn't consider any under saddle work until after the age of three. Even then, if the horse is highly undeveloped, I would post pone. It isn't worth the deterioration and interference with a horse's growth in my perspective. At three, I would consider desensitizing by placing a saddle on the back and exposure to tack. If the horse has developed well, I would break at 3 1/2 with basic walk, trot and canter. Nothing, by any means, grueling.
My horse was finally gelded at the age of eight and then trained to be ridden. Eight is a very long time to wait, yes, but now that he is at the age of 22, I am very happy that he didn't have the unnecessary and stressful training too early - his health reflects that.
Just my opinion.