The issue I see with some of it is that there is no solid way to track late age lameness and discomfort strictly
to the age they are started. It is just as feasible, in my opinion, that a horse who is started at age 3 or even 4 or 5 for that matter, could end up with arthritis, stiffness in their back, weakness and general discomfort if they are ridden poorly all their life.
I have been on my 2yo about three times, totaling less than 10 min in her life with a rider. I work her lightly in side reins about 3 days a week. When I say lightly, I mean that she spend only 3 or 4 minutes with the side reins attached and that covers going both directions and her lounge sessions are less than 20 minutes all told to protect her knees. I do a lot of in hand work, leading, line driving and exposing her to weird obstacles. I will not trot her under saddle until spring of her 3yo year and asking for canter will come as a result of her being mentally mature enough to handle it. I am guessing that will come close to the end of summer her 3yo year based on how she has learned up to this point. I find it hard to believe that this level of work and slow methodical conditioning is going to make my horse a cripple as some people have claimed.
Conversely, I personally know a horse who was not started until he was 5. However, from the first day, he was ridden with his head pulled up, hollowing his back and putting strain on his neck. There was no forward drive, no relaxation and certainly no concern for his top line. This horse was totally shot by the time he was 15. The poor guy was so arthritic he was unridable. He was put down at age 17 because the pain was so bad he became aggressive and dangerous.
I would LOVE to see a study done where a handful of trainers were given horses to start at different ages. The horses would need to be as similar in structure and temperament as possible so that the ONLY variable would be the age at which they were started. Track these horses over the course of 20 years and then determine what age would be optimal to start a horse for health and performance.