I have three horses; a QH, a TB and a WB. I’ve owned all of them since they were at least 1 yo so have/will have them all broke to ride.
I rode the QH in the spring of his 2yo year and showed him that summer under saddle in AQHA and 4H events. His 3yo year he went into training to be a reiner and reined for about 5 years. He’s 22 now and I foxhunt on him. He’s never been lame other than some weird bump on his knee that just looks bad and prevented him from doing nice spins thus ending his reining career.
I had the TB broke to ride at the end of his 3yo year. I took him to a couple small local shows his 4yo year, but didn’t do much with him until he was 5 or 6. He’s 8 now. Sound and showing in the hunter classes. It took him much longer to mature mentally than the QH so I took things much more slowly with him.
The WB (technically she is 1/2 TB, but is registered as a WB) is 3 and I won’t have her broke to ride until the end of her 4yo year. I’m breeding her this spring. She seems a little further along at 3 than my TB was at the same age, but definitely not as mature as my QH was at 3.
In my experience the QHs can take the push mentally and physically to start riding at 2 much better than TBs and WBs. They are also physically mature sooner. My parents raised QHs and I don’t recall any of them gaining much height after their 3yo year, where as my TB grew until he was 6. My TB is mentally at 8 where my QH was at 3. There are, of course, exceptions. A lot of the QHs bred to do the English classes are 3/4 TB these days so will mature more like the TB than a “normal” QH.
I try to ascertain where each horse is mentally and physically and go from there with their training. I’m in no hurry and I want them to tell me when they are ready to get pushed harder. It’s fun and challenging and most often also rewarding!