This is an interesting article on how the brain develops, and whether it ever finishes. There is also an interesting discussion of teenage thinking and emotions.
The human brain reaches its adult volume by age 10, but the neurons that make it up continue to change for years after that. … The pruning in the occipital lobe, at the back of the brain, tapers off by age 20. In the frontal lobe, in the front of the brain, new links are still forming at age 30, if not beyond.
Source: You’re an Adult. Your Brain, Not So Much. – The New York Times