for immigrants brought into the country at age 15 to 20.

There is no one child per person policy here ,nor do immigrants have one child , they apppear to have 4 to 7. Remember an immigrant has no family here and therefore their children become their extended family, and in most countries children are considered ones old age retirement policy.

Thus the increase in US population under the current policy would be astronomical and into the several billions within 100 years unless something effects that,disease or starvation.

Fifty or sixty years to wait for an increasing population to start decreasing sounds like a disaster.

Well at least you understand the mathematics of the problem. Most here consider overpopulation a miracle of some sort.