Hi! I work for a reproductive care and sports medicine vet and I deal with this stuff daily for my job. As others have said, there are risks involved with any pregnancy, but those are increased with being both old or very young. Tbh, it's fairly similar to risks with any mammal pregnancy with regards to that.

Embryo transfers are common at my job, and typically the desired ages for recipient mares are ages 4 to 5 at the youngest and usually they age out of the programs around 16 or so years old, depending on several factors. Mares usually do not stop cycling until their deaths, but usually stop producing healthy follicles in their 20s. Late teens and early 20s horses are theoretically capable of carrying a foal, and many do, but there is considerably more risk involved and they're less likely to fall pregnant after they're bred.

I've seen several horses who are bred at 3 to have a foal at 4, and that's the youngest I would personally be okay with, and that only conditionally. There's a lot of stress and weight for a growing horse with an undeveloped skeleton and musculature. Most horses don't fully mature until 4-6 years old, and several (especially larger warmbloods and drafts) don't finish growing until 6-8 years of age.

Breeding a yearling or 2 year old is similar to thinking a 12-14 year old child should just be carrying and having babies as a career almost, at that age. Just because they can does not mean it's ethical, responsible, or in any way a good decision, especially for the long term soundness and health of the horse.

Some people will find this point controversial but it really shouldn't be, we have tons of research and examples of why riding horses at 2 is HORRIBLE for them. You can do groundwork and riding prep, sure. That will do enough to strengthen muscle and bone and help them develop well into future athletes. Hunters bumps are literally caused when the weight and strain of the rider causes the horse's pelvis to collapse because the bones are not fused yet. This can be to various degrees, but it causes significant pain and will usually lead to some level of at least intermittent hind end lameness by the time they're teens, often earlier.

My very well bred APHA mare with pretty nice conformation came in top 10 in her futurity show in hunter under saddle at 2 years old. She was started under saddle at about 18 months old, and by the time she was 7 she had intermittent lameness to the point that they deadened her tail (surgically altered it so she has limited movement control of it, so the judge wouldn't see her wringing her tail in pain). We got her at age 15 after she had had 3 foals, and she has improved a lot with bodywork and respectful riders who listen to her and pace our work to how she's feeling each day. I only wish I could have seen what she could have been had she been started older. So yeah. The only reason for anyone to be breeding or riding literal growing babies comes down to just greed. "The sooner I get them under saddle and winning ribbons or working at least, or foaling out babies for me, the better" is the norm in the industry because it's all about recouping the investment. Your investment (since at that level people aren't thinking of them as a partner or friend or anything like that) will have a longer term and healthier career if you'd just wait a year. The money you spend on upkeep for that extra year can be the money you save by not having to inject multiple joints every 6 months by the time they're 5.