Should we ride two-year-old’s?
Think about it this way; we vertebrates are born with most of our spine structure actually made of cartilage, which is soft and yielding. As we age, ossification, or calcium deposition, fills this in until our skeleton is mostly solid, stony bone, which can hold weight. Like on the TV show “Bones” you can tell the age of a growing skeleton by the amount of this ossification.
A human being closes growth plates around age twenty, give or take a couple of years, and depending on gender, etc. A young growing horse finally closes growth plates, or his skeleton becomes all bone, at about age 5. The last growth zones to fuse are in the spine, the area where we sit when riding.
I guess, for my money, since I weigh almost 200 pounds, and my saddle adds another 35 or so, I’d rather sit on a more mature spine.
You hear about x-raying colt’s knees. They fuse at around age 2. But I am more concerned about his back. So we start the early work at age 3, with intermittent, limited riding. At four we do a lot of slow work, and maybe a little trail work, saving the real training for five-year-olds.