How young

What is the youngest you can safely and healthily breed a mare? I've known people to breed at three-years-old...?

quite a few people in europe breed their 2 year olds for them to deliver as a 3 year old, but personally, I wouldn't breed unless the mare was minimally 3, preferably 4. any younger, and they are still babies themselves and needing nutrients to grow and develop on their own-without having to worry about a baby

I think the earliest to breed a mare would be at 3 years old and they would foal at 4.

I breed at 3 unless I'm showing a three year old then I breed her at 4.

My vet had told me that she likes people to wait to breed until the mare is 5 years old. I think that way they are done growing themselves.

I would say 5 is a nice age. That way you know how the mare ultimately turns out, plus she is all done growing.

It's really up to you and your vet. I know a mare who was bred (completely accidentally, btw) as a yearling and delivered a healthy foal at 2. Both her and the foal are completely fine.
Talk to your vet, preferably if they are a well spoken of reproduction vet and decide. It depends on the mare.

I never bred my horses before they turned at least 3, usually pushed for 3 and a half. Breeding is risky business but breeding a horse that isnt even done its growth spurts is bad business. Imagine having a 12 year old kid pregnant (not that it doesn't happen regularly). You can imagine all the physical implications of that one.

My general rule is 3 but really its a horse dependent decision but the older the better (on the young end of the spectrum). Hope it helps.

In the wild they would be pregnant pretty much once they come into season wouldn't they?? Not sure exactly how old that is...

But I agree with Cachedawntaxes... the more they've grown and matured, the better :)

I would say if you were to breed her at the VERY earliest, 3 years old. But I personally would wait until 5 and foal at 6. That way the mare has more time for her body to fully grow and mature safely. But that's just my opinion.