I often just wonder how long I have to bubble wrap my horses to avoid being told I'm a horrible abusive horse mom for riding them at two.
I don't argue that growth plates aren't closed, I'm just saying that I'd rather start my horse and get them going in life then wait and wait and risk more problems, or something happening.
Noting also that the type of horse I have is bred specifically to be started quickly and moving along. I do think there is something to be said about strong work ethics, conformation and genetics. Your everyday backyard horse who takes six years to reach full height and width isn't going to be as structurally sound as these guys below, who were two at this photo, and full grown by three - Maybe four in the roan's case, and bred to be balanced a lot more naturally than say, an arab or a gangly warmblood.
The reason is simple, horses who did not mature quickly mentally and physically were not bred because they couldn't handle the work and took too long to prove themselves, so therefore there was less reason to breed them.
I've said this about a hundred times on this forum too,but there's a "don't be stupid" clause built into this. Yeah you should probably not take your two year old, jump on, whip and spur and kick and run it into the ground. You shouldn't ride it until it's foaming sweat and put it away wet. But this goes for all horses, not just colts. For me, I ride until just before they will be uncomfortable. Enough to stimulate strengthening of the muscles so they get more fit, but not enough to cause any real stress. It is not hard to avoid doing damage, you just have to possess a lick of common sense. I understand how that may be hard for some people and so thus caution on the forum is understandable, but I am the realistic and practical voice in this situation so I will provide advocacy for the practice here.
A typical ride for my two year olds is not dissimilar to a ride for my elder horses, except I have to remove some things they won't know how to do yet and add them as they learn. I warm up all my horses by walking five laps each way, trotting five laps each way, and then on colts I usually get them loping circles each way until they are relaxed. Sometimes that's two laps, sometimes it's ten. Depends on the colt. At that time we will break back down to the walk, do some suppleing exercises, or if the colt has a particular job ahead of him (i.e. barrel racing or cutting), we will end the ride doing something related to that. I.e., ten minutes on the mechanical cow, or some walking/trotting through the barrel pattern. If he's going to be a trail horse, we will immediately leave and go for a short trail ride out into the back forty.
This allows the muscles to really warm up and cool down. Similarly, I do constant body work myself, so it is easy for me to see muscle tension and any other discomfort as soon as it arises. Their legs get cooled off and they get fed excellent supplements. Only the best. Again, I believe it is my responsibility to give the horses the best chance at life and the best possible training they can receive. So therefore, this is how I do that. The fact that my horses keep selling and they keep sending me things to start and train tells me that I must be doing at least something right.
Here's a few photos of some fit two year olds after about 30 days riding each.