You've missed a vital detail I believe. Even after birth, the brain must still develope. Some might say that a brain is never developed, but on the whole, the baby brain doesn't have enough time to mature into functionality. The earliest functional brain would be around 1 or 2 years old. and even then, the yeerks have developed minds, when combined with the babys, it would imprint. creating a human copy of the yeerk. But I'm just guessing.