ACKTCHALLY.
The reason for that is a weakness in human gestation. Because the mother wouldn't survive 10 months of pregnancy, babies are born premature.
Horses, however, don't have this weakness and gestate long enough that the baby can walk within hours of birth.
Now.
Since Centaurs have horse... organs, the baby would be carried to higher maturity before birth. I'm going to say its equal to maybe a 4-5 month old. Not terribly strong, but enough to hold their heads up.
it's just supposed to be funny. Stop fixing it. 
