When I first got Minis the common believe (1999-2000) was breed them at two and keep them bred every year. The whole 'cash cow' mentality as those mares are producing product, blah blah blah.
I bred two at two, one aborted one didn't take (back then). I thought about it and the correlation I give people is teen girls can get pregnant but should they and do they need to?
I figure the same goes for my mares. A healthy broodmare even started at four-five has at least fifteen years to carry foals, so IMHO plenty of time to contribute her genetics to the Mini pool.
The earliest I breed now is four, but most are close to five (the under 30's are five+). I've bought bred three year olds, but I personally prefer to wait. I also don't breed back/back in general. I have but not often.
As for what Jen mentioned, maidens can be a PiA, I've been fortunate as the majority of our maidens have taken to motherhood extremely well (their dam's were good dams) with little fuss. But I've had other maidens that were bred at 2-3 and they were a gigantic pain, not all but enough of them. They didn't know what was going on and most definitely didn't want that little alien 'back there'!