I personally have never bred one of my own mares before the age of four, but did once breed a three year old outside mare. They had wanted her bred as a two year old, but I refused...so they waited.
Horses of any size... "industry standard" is to breed the mare at three years old, so she foals at four years old.
Some people DO breed at two, but I am among the majority that feel it's too young... two is too young to ride and compete and breed... let them grow up. I feel that breeding them at two is like treating horses more like livestock... treated more for greed than what is truly best for the animal.
You will likely get lots of opinions... but that's mine.
I bought a nice little mare last fall at a sale that turned 5 in April. She had her 2nd foal in March, at 4, almost 5. I bought her bred (I did not breed her) and I also bought her last year's foal at the same sale. She was bred the first time at 3 and foaled at 4 and bred again at 4 and foaled at almost 5. She is 5, but she still seems like a "baby" herself. She won't be bred again for 3 or 4 years or more. If you have to, have to, have to (and who has to?) breed a mare, please wait until she is 3 and foals at 4, BUT I personally don't breed mares until they are 6. I have had (big) horses still gaining height at 5 and 6.
I personally wouldn't breed until the mare is at least 3. Even longer wouldn't hurt. When I say three, I don't mean in January, but their actual birth date.
I breed my mares at three....I have bred, by accident, at two (to foal at three) and not had a problem, but I would not wish to do it intentionally.
I always bred the Arabs at two, and I never did have a problem, but there was a different rationale behind it, as they went on then to be riding horses.
I have also known mares that were maiden at 16 foal without a problem, but, again, this is not something that I recommend!!
We wait till they are three, and sometimes 4-5. Depends on the mare, and if she is showing or not. My Heart filly that died last June had just been bred for 1 month when she died, and she was 5 then. This year, her dam had a very tiny filly with much the same personality, and who knows, she may become my new Heart filly! She seems to be on her way.