If we need more young people in our society in the USA, this is actually the easiest problem to solve -- just open up immigration. As long as lots of people still want to come here (not guaranteed to last forever), not having enough people is a problem only of our own making. If only most of our problems were so easy to solve.
It's only a short term solution, as all countries around the world (even those third-world ones, with current fertility rate above 4.0) are experiencing lower birthrates every year. See the movie birthgap