I think Ahmed has a point, some people never reach a level that we would call 'Mature'. This makes the issue of assigning an age to the mindset a tricky one. I personally believe that someone can not truly be mature before the age of 16, and that is a rather liberal number on my part. I chose 16 as an absolute minimum due to the amount of life experiences and biological development an individual requires to become self aware. That is to say maturity is simply an extension of awareness and as such a broadening of the theory of mind.