For the first six months of postnatal development, the baby's thymus is stimulated by secretions in thier mother's breast milk. This allows the mother to pass along whatever immunities that she has developed on her own, giving her child an even greater chance of survival.
After about six months, the secretions stop, and breast milk is about the same as formula. So there's no need to breast feed the kid until he's sixteen in the hope he becomes a super hero or something. (more likely than not, if you breast feed him until he's sixteen, he's going to be one of those creepy kids who always quotes Nine Inch Nails lyrics under his breath when people give him crap for being such a weirdo)
-kv