[DanaC]

Originally Posted by Kizzy

How could anyone know that for certain?

There have been quite a few studies into this. The last part of the brain to develop fully is the pre-frontal cortex. This is the part of the brain which is responsible for the complex processing of information, ranging from making judgments, to controlling impulses, foreseeing consequences, and setting goals and plans.

It used to be thought that the brain was mature in adolescence. More recent studies have shown that it can take into the early 20s for this part of the brain to fully mature.