I think that it is best to go slow with young horses. Take the time while they're still maturing to gI've the a solid foundation on the ground. The when you start to break the to saddle (In my opinion 3 is the ideal age to do that) some of that work will transfer under saddle
For quarter horses it's more accepted because their joints mature faster and they're more hardy, but I agree that it's still unnecessary to put them through that if it even MIGHT be hard on them. Warmbloods joints don't solidify until there even 7 years old, so even working them at 3 is not ideal if you want to do what's best for them. However, some studies show that muscle building helps them be strong as they solidify, but I would look at the "work" as only rehab work until there full grown and more filled out as far as you can tell.
Depending on the breed, you can start riding at different times. With Quarter horses, I only start them when their legs finish growing (most of them right around 2. I do talk to my farrier and sometimes the vet to make sure they agree with me. I do not do anything more than walk and trot for more than 30 minutes for the first couple months. You want a good foundation so starting slow is best. I am not truly riding my horses when they start. I more get them accustomed to someone on their back and what rein and leg cues mean. I definitely know what its like to have a horse that was started to soon. I have one of those and I had to quit everything but pleasure bc her knees where so bad (she is only 13) I do have a 3 yr. old that I have just started to do much more work with. I have only now started to do canter/lope work.
It is completely wrong to do so. Horses at this age have not completely developed their bone structure and tendons and ligaments yet. You would do harm and in later years your horse will have difficulties with staying sound
The horse's end height determines how long it takes for them to finish growing. A regular equine, around the size of 14hands, only takes 5 years to develop. Whereas warmbloods and thoroughbreds that are around 17 hands take 7-9 years.
Horses basically grow from the ground up. Leg bones grow first, then hips and shoulders, then face expands, with the vertebrae taking the longest as it is covered in growth plates.
No.Period. Horses grow until they are 7-7.5 yrs old.
These days, horse dealers are too eager to make money and make horses work to much too soon, which can lead to nice vet bills with horses over 12.
Before two is too early, especially if someone is actually going to be "riding". As Tata said, this can be detrimental to their growth in the long run.
Not recommended, especially for larger breeds. The leg bone's growth plates are not completely formed - too much work can damaged that and cause bone issues. Light ground work till they are 2 or 3 is better.