In other words, when the child turns 21, he first needs to claim his US citizenship based on the fact that he was born in the US. Subsequently, as a US citizen, he is entitled to apply for his parents' residency. He can also apply for his siblings. But the processing time for family applicaitons are quite long, I think and the person sponsoring residency of the family should have sufficient income to support them financially when they arrive in US. At the age of 21, this might be a bit far fetched. So it is not as straightforward as it may seem.

Mathematically, it will be 22+ years if one is to opt for residency by bearing a child in the US... :-)

Or 19+ years as per americanboy if that makes you feel any better.