Teen marriage rates (15-17) are alread exceptionally low. One would expect as laws are enacted at the state level to raise the minimum marriage age to 18, these declines should continue.
I don't dispute that the teen birth rate is much lower now. I'm trying to estimate out the counterfactual "what would the US birth rate be if teen pregnancy was (say) the same as 2 decades ago" ? Currently it's 1.66.
But the back-of-napkin math is hard for me to figure out from the given "births per 1,000" women per different age group.
