US federal population projections don't foresee a big change in 18-24 year olds into the future [0]. Though generally there's a trend of a slight increase in foreign born populations over native born.
I would think even more important to universities should be the raw number of high schoolers and high schoolers’ parent that are questioning the ROI of universities, especially private universities.
The ROI of elite private universities like MIT, Harvard, and Stanford are still very high. The private universities that should be worried are the ones that don't have an elite reputation but still charge the same tuition (or higher tuition, if you account for financial aid) as the elite universities.
For the field as a whole I guess, the rate must matter, right? Like, if the birth rate increases, then a higher percentage of the population will be employed in teaching related jobs, because there are more kids per adult.
[0] see particularly table 2 https://www.census.gov/data/tables/2017/demo/popproj/2017-su...