Around the age of 11-12 your brain should be close enough to be fully developed your on crack bro the brain grows until the early to mid twenties the frontal lobe isn't even close to done maturing at age 11 -12.
Just after puberty, this time is when the brain is 'metaphorically' rushing around trying to sort everything out before you get too old. After this has happened the brain has a hard time trying to...
not until 23-25 years of age. it is very important that external hormones are not introduced before this age(ie testosterone or growth hormone). women will usually reach this point before men.
The brain does most of its development in the first 20 years of life, but new evidence suggests that it continues to develop, even into old age as long as it gets plenty of mental 'exercise'.