They don't finish growing til they're 5 years of age.
Competitors typically start em when they're 2 to turn em into money makers asap.
I started my quarter on some ground when he was 3 and didn't ride him til he was 4.
The vet will tell you that the knees close at 2. But the horse's spine is still developing, he's still growing, and doesn't fully mature til he's 5 years of age.
The bigger the horse, the more time he can take to mature. I've got a draft that's 6 and ready to be ridden (light). Drafts, for example, don't finish growing and maturing til they are 6-8 years of age.
People get in a hurry, because it's a lot of time to wait til the horse is truly old enough. So, if you choose to start em at 2 or 3 be aware that you may damage them permanenty if you're not careful, because they are not fully grown (not just height wise, but also length wise...the vertabrae)....yeah, some horses are started quite young and are sound as ever, but there are plenty more that are arthritic and have other issues.
Just be careful.