Why do 21 year old sons treat thier mothers as if they don't understand anything?

[Matty]

Because each generation learns more information than the last, and the younger generation feel as though their parents know nothing compared to them. It's not always true, but quite often is. The older generation also hold a large amount of information which is no longer of use to modern life, while the new generation are pr0 with the 'ole twitter and facebook. The next generation, however, will laugh in the face of the twitter experts.

Let's be honest, you don't learn much from the age of 21 onwards compared to previously and it's only human to start forgetting the stuff you learned from the moment you stopped learning it. Unless you work in a very academically challenging area or hobby, you're unlikely to know as much as you knew on the day you left school/college/university.