According to research carried out at University College London, the human brain only matures fully when you are middle aged.

(In the research scans showed that the prefrontal cortex – the area just behind the forehead – continues to change shape into the 30s and 40s.  The prefrontal cortex is a key area of the brain and is often thought said to be key to what makes us human.  It is said to be involved with decision making, social interaction and many other personality traits.  Which goes to prove my “teenagers are another species” theory.)