Evolution of the Human Brain

My almost three year old experience with my nephew tells me that babies are smarter than most people take them for.  They are people also, only mute for a very brief period of their lives, so others would mistake them for dummies instead.  Once they start talking, they don't take anything you say for granted but rather start questioning everything: they keep asking why this is wrong, and tell me why should I listen to you rather do what I want to.
Facebook has also exposed me to teenagers I once knew as kids, and the idiocy I keep seeing in their posts is just preposterous.  It seems as though peer pressure and going with the flow is much more stronger than the urge for stopping one second and questioning whether this flow makes sense or not.  When they were kids they were just as skeptic as my toddler nephew, now all they do is keep on forwarding chain letters and having weird posts all over.
The question is, is it part of the human growth that the brain declines into the pits of idiocy, or does it just take a break in the teenage years of one's life, and then picks up again?
I'll keep a watch and hopefully I may find an answer in a few years time!