I totally agree with this--if it's a mere numerical difference, with no underlying process that changes in a nonlinear way, it does seem bizarre that one value would be totally cool and a slightly larger one would be tyrannical.

The thing is, human development is not just a steadily increasing number. Neural development proceeds in a variety of key phases as different neurons are born in waves, different neurons send projections to various other regions of the brain, and so on. Between 15 and 24, these things can go from "completely absent" to "entirely in place", which turns a quantitative change into a qualitative one. For instance, the difference in reproductive capacity between an 8 year old and a 16 year old is usually not a factor of two.