While I agree the point of proportionality is often ignored, the US population isn't what you want to be looking at, especially since—as far as I'm aware—there is more diversity within the younger generations in the US. You would want to be looking at the demographics of students applying in a particular year to university and looking at the disparities in who is accepted (and, of course, looking at how this plays out at different universities and so on). You also want to look at the demographics of the age range at which most attend college and see whether there are any disparities in those who aren't applying to college at all.

While many may not want to go to college, there may be those that would be but are facing barriers—perhaps poor schooling, lack of outreach and guidance, discouragement from educators or a lack of financial support—and you would want to see whether this is affecting some groups more than others.