I'm 16 and I've been playing MMORTS games for 6 years, starting on Tribal Wars, which is arguably more competitive. I started leading 4 years ago. I've led top 5 tribes (alliances) for the last 3 years. I led a Travian alliance that won the server 2.5 years ago. I've been rank 2 on a TW .net world with over 700 villages.
Age does not determine anything. Maturity doesn't necessarily, either - there are a lot of great players who are incredibly arrogant or egotistical. If that's not a product of immaturity I don't know what is. A higher 'mental age', as it has been put, will obviously limit mistakes but I don't feel that the MMORTS template and basic arithmetic is the exclusive realm of geniuses, or even of that of people of an 'average' intellect.
I put it down to experience of the game and dedication.