I think it's even slow for high schoolers. I didn't practice that much and ran 100m in 12.5s from rest at my peak. 4s slower is snail pace. I think most in my class could run that fast (or slow).

I think height matters for speed as a fit 6ft+ would easily run way faster than a 4" 8' fit person.

I agree. I ran mid 16s in 8th grade, and was in the 14s in high school, with the only training being whatever we did in gym class. But I do also look at the sheer number of overweight kids these days and figured, well maybe mid-16s is actually a reasonable average point.