One thing that does help the GOP a little bit among the youngest voters though is this: Didn't Quiverfull really take off in the mid-'90s?
I remember seeing an article about 10 years ago that said the birth rate among conservatives was much, much higher than among liberals. I don't know if this was true before that era, and I don't know if it's still true today, but that was certainly the trend 10 or 20 years ago. That was really the entire point of Quiverfull.
Unfortunately, the right-wing media scapegoats poor welfare recipients for having 1 or 2 kids, while praising right-wing Quiverfull types who have 15 or 20.
It was probably always this way since at least WW1. My grandmother's mother was really religious and had 12 kids but that doesn't mean that sort of phenomenon dominates the discourse.