For most people this is when we really begin to come into our own socially and intellectually. SometimesP can probably give a complete breakdown but in simple terms my understanding is that our brains dont fully develop until mid 20s. Our ability to do abstract thinking etc is just limited before this. Also by our twenties we have usually developed a pattern of life that allows more structured and productive thinking. Hormone levels arent in the same turmoils as when were teens. Our twenties are really the first time we can start to understand and make intellectual connections across wide areas.

This is a good time to be in school because you are also still flexible in your thinking and can develop life long skills more easily than in your later years (thirties, forties etc).