Since they're born from the horse end, they could have horse like gestation. So they can probably hang out in the womb about 2 months longer. Not a huge difference for humans. However if we're assuming they are a product of evolution, it would stand to reason that they could have accelerated growth compared to humans. Add in that centaurs are generally omnivorous, you get a higher density of nutrients consumed. It's possible that the longer gestation, plus more nutrient dense diet, plus some evolutionary shenanigans (centaurs maybe did have toddler heads and horse bodies, but those just died out), they could end up being born with enough back and neck strength to keep themselves upright.

But we're also talking about centaurs so the answer is probably fuckin magic I guess