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
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.
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.