I have enjoyed starting many horses under saddle over the years. I've started two year olds, I've started 20+ year olds, and I've started many ages in between. Years ago I got a phone call from a rancher in Wyoming, who had "30 head to get broke" that year. He was hoping I'd come to his ranch and over the summer get them to be green riding horses. I politely declined, as even at that time I didn't travel to outside ranches to train horses, and I wouldn't leave my facility here in Colorado (to likely go get killed trying to ride baby horses who had never been touched). Now when I get phone calls asking if I "break" horses, I reply saying, "I do my best NOT to break them. I do start them under saddle."
This diagram showing the approximate ages when growth plates mature in horses. There is some variability in when the growth plates finish maturing, as geldings tend to mature a little later than stallions and mares, and the larger the horse (17h vs. 14h) the later they mature. I grew up hearing the conventional wisdom that you should "wait until the growth plates close in the knee" before working the horse really hard. Now that we know more about the growth process, it makes more sense to me to wait longer.
The growth plates closer to the ground mature first, and the last ones to finish are at the top of the body, in the back and neck. Since when riding we sit on his back and the horse needs his neck for balance, it makes sense to wait until he's older to start riding him. To be honest, the simplest horses I've started have been in the 6-10 year old age range. I suspect this is not only because they have had time to mature mentally, but they also are physically capable of doing what I ask.
Often these horses were purchased as youngsters and then life got in the way for their owners, and eventually they realized the horse was an adult and had no riding training. I really enjoy working with horses from these situations, because they often have had time to grow up and hang out with a herd learning social skills, which makes starting them under saddle much easier. Baby horses have the attention span of a flea, but I can work with an adult for the better portion of an hour without losing their attention.
In my barn I have several larger Thoroughbreds. Highboy and Dewey are currently my tallest, Highboy at a half inch over 17h, and Dewey at 16.1h. Highboy continued getting taller until he was eight, he finally hit 17h in his eight year old year. Dewey grew his final half inch of height during his eight year old year. They both had been ridden as youngsters as they were both off the track, but hadn't been ridden hard as they were abysmally slow. I restarted each of them gradually, and didn't ride them hard until they were more mature. I think that's partly why Highboy is totally sound and loves his job eventing at 12 years old, and Dewey is such a nice mover despite his somewhat wonky conformation. Anecdotal evidence to be sure, but I can list many just like them who have been in my barn for years.
In my own ideal situation with no time constraints (no futurities, no young horse or future event horse competitions, no need to push the horse in its training to get it sold), I prefer this tentative schedule for starting a youngster and avoiding overuse injuries:
Yearling: Come for 1 month of in-hand training to learn ground manners when leading, how to stand tied politely for grooming, loading in and and out of the trailer, picking up feet for the farrier, standing quietly for the veterinarian. Introduce obstacle work such as bridges, tarps, hula hoops, anything else found at the dollar store to desensitize the yearling to new things.
Two years old: Come for one month to continue ground work. Refresher on yearling skills, plus learning to free lunge with voice commands, how to wear tack from surcingle/pad to saddle/breastplate, free jumping to assess natural ability and whether he likes it. May begin attending competitions as good exposure to "real world" situations.
Three years old: Come for a month to review previous training. Add to it minimal lunging on a long lungeline, introduce wearing a basic snaffle bit, introduce ground driving, clicker training to finesse obstacle work, begin ponying while I'm riding one of my older horses.
Four years old: Come for 3-4 months to begin riding if the horse is ready. If the horse is not ready mentally or physically, repeat three year old work for one month instead.
Five years old: Come for 3-4 months to begin riding if not previously started. If the horse is not ready mentally or physically, repeat three year old work for one month instead. Starting under saddle includes learning to carry a rider at the walk, trot, canter on both leads, halt, back, turning, and riding both in the round pen/arena as well as out in the fields and on trails. Long walks with lots of gentle hills develop the horse both physically and mentally.
Six years old: Horse can be put into full training if mentally and physically ready. This includes reviewing everything from year five, plus introducing small cross rails for jumping and low cavaletti work. Sliding and spins may be gradually introduced at this time.
Seven year old: Horse can be in full training if mentally and physically ready. Jumping under saddle with a rider over various obstacles both in and out of the arena, gymnastic work, with spins and sliding stops. Cross training in various footing, sand, dirt, grass, rocky trails, hill work.
Eight years old: Horse can really begin to compete in his preferred discipline.
I recognize that my "ideal" schedule is a generous one regarding timeline. There are very few horses who are allotted this much time to grow up and learn their job. It's understandable from a financial perspective. Who can afford to give their horse this much time and training before starting to compete? What about all the young horse competitions, such as futurities, maturities (still for young horses), young event horse, future event horse, or FEI young horse dressage competitions? What about having the young horse move quickly up the levels as a way to prove its athleticism and value, as well as the trainer's value?
In this industry, a traditional way trainers try to make money is by getting a horse, putting some training on it, and selling it for more than they invested. People tend to think that the horse is "better" if it's farther along in its training at a younger age, that a trainer is "better" if they have horses who are quickly moving up the levels. People want to purchase young horses with lots of training. This may be the standard in the equine industry, but is it the best thing for the horse?
So many issues, both physical and mental, can be from overuse. Of course horses are flight animals, and some do their very best to injure themselves as often as possible. But if I can avoid problems by starting my horses later and slower, I'll choose long term soundness for them every time. I'm extremely fortunate to be in a position where I could take years to get Highboy jumping around a cross country course with prelim level questions. I'm also grateful that I was able to develop Dewey slowly and correctly, so that his superficial scars and injuries are from accidents while playing, and nothing that will limit him athletically. I recognize this is not an option for so many people and horses, and I'm extremely thankful to have the options I do.