Recommend you listen to this [1] interview that I heard earlier today. It's by a Stanford neuroscientist, discussing why and how the teenage brain leads their owners to be impulsive, addiction-prone, and just generally idiotic.

The TLDR: they really don't know exactly what they're doing, because the physical substrate for their cognition can't yet support those functions.

[http://www.npr.org/programs/fresh-air/archive]