I'll admit that I've never worked with a young horse and so have never experienced the "when to ride" dilemma in real life.
However, I have been around horses that were constantly "broken" and ridden hard by the age of 2 (the horses from the summer camp I worked at for 4 years) and I can tell you that nothing about their attitudes or how sore their bodies constantly were by the time I was working with them (10-15yr olds) makes me want to condone riding a horse that young.
Those poor horses, the "oldest" one I have got to be around was 20 - lame as all get out and HATED humans. She looked about 30 and there was nothing about using her in our riding program that I really liked. She was super broke and a really easy ride, but at what price? Just carrying small (under 100lb) kids all day obviously made her tired and sore.
The 15yr olds were, as a group, unreliably sound and were constantly playing up because they stepped wrong/twisted something/etc and most of those horses had to be given a week off every couple of weeks if I wanted the kids riding them to survive. Swollen joints were par for the course and it was very unusual to have a horse that wasn't obviously in some sort of pain - joint pain or otherwise.
I just couldn't handle it anymore so that's why I'm not working there this summer, before anyone asks. :)
Anyway, I think that if a person is able to accurately pick out the broke early horse from a group of 15 year old horses after seeing those horse interact with people and move around - you have a problem.
The difference may not be in the lameness or not of the horse but there's just something about a horse that wasn't allowed to have a "childhood".
I liken it to teenage girls having babies or not. Sure, a 14 year old can get pregnant, keep her baby, and care for it as best she can, but, we all know the answer to this, how well is that young mom and her baby really going to fare in this world? That girl "lost" her teenage years when she was going to be truly developing who she is and what she stands for. Now, she's a mom and she stands for raising her child. She's going to have a hard time going to college and she might even have difficulty finishing high school. It's possible for her to do those things (just like it's possible for a horse that was broke early to end up fine) but it's going to be about a bazillion times harder for her.
A 2 year old (or younger) CAN be ridden but is it reeeaaalllllyyy the 100% best option for that baby?
For me, I like my horses old and sound. Take Lacey, for instance. She's 27 and still going strong. No lamenesses, never been injured - no swollen anything ever even though I ride her hard for her age, does get sore sometimes but reasonably so, still loves to run - runs around the pasture often, etc. Right now, the only thing changing Lacey's way of living is her failing eyesight. If she could see 100%, there'd be no telling her apart from a 10 year old. Depending on the 10 year old, you might even be tricked into thinking she's younger than the 10 year old!
She was started late, retired at 12ish for "behavioral issues", and only brought back at 23 when I got her. Now, I'm not saying that all horses should be retired at 12 to have active senior years (though I'm sure it helped us) but I think that breaking a horse at 4 or 5 is the key to creating a horse that'll do well as a senior horse.
That's just me though, I figure wait now to get ahead later.
It probably depends on the person. I personally don't see the logic to it, but people sometimes seem to think 20-somethings are plugs and therefore don't want to ride them. In that case, I can see twisted logic in risking "using up" your horse young because if 15-19 is "the end" of when you want to be riding horse, why does it need to be riding sound after that?
I really don't like that logic because I adore old horses, but I can see that being reasonable in someone's mind.
Just my thoughts. :)
Fabio - 13 year old Arabian/Lipizzan gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.