Riding a horse won't stunt it's growth. Improper feed and nutrition can. Horses all grow at different rates of speed, QH's grow height-wise for the first 5-7 years and then width-wise they continue to grow from ages 7-10 years.
It really does depend on breed, mind, discipline... but also truthfully, if you don't believe in breaking a horse until age 3 or 4, then don't... break it when you want to... just know that 5 is pretty much the end of your window....
I have to disagree with farmpony, sorry about this one.
Beau our six year old Stallion was started last year as a five year old and we are just finishing his training now, he only had the basics last year. And he is learning way faster then alot of 2-4 year olds that I have seen started. North is also a 8 year old and was just started this winter, even though she bucks (once in awhile) so is learning her rein aids, leg aids etc. so quickly. So I totally have to disagree that after five you lose your window.
I might as well go ahead and say.... I show at the VAHA shows (Virginia Arab Horse Association) and I have noticed that THOSE PEOPLE have started the arabs as late as 10 but you really can't do that with every breed....
...throw's a cup across the room and takes a deep breath....
YOU HAVE TO AGREE WITH ME BECAUSE MY OPINION IS THE ONLY OPINION BECAUSE I AM THE PRETTY PRETTY PRINCESS!!!!
JDI's message was short and sweet and a very good answer. I know a few trainers and people who say to take horses out at 2, teach them to walk, trot and canter then throw them back out in the field until they turn 3. They are supposed to be more responsive and easier to train because they have learned good work habits as youngsters.
In a way I can see how that would work well, but at the same time I don't know of a single 2 year old that is developed enough to be put thru so much stress while being so underdeveloped.
I think it depends on the horse and the type of riding you're going to be doing with it. I personally would never start a horse before 2 years of age. Both my horses were just over 2 when I started them, and they turned out wonderfully. My first, a mare that I bought as an untouched yearling and sold last year at the age of 15, never had any issues related to being started at two.
I have seen some 2 year olds that I definitely would not start til 3.