It's a very subjective question as you may have younger people who can control their emotions better than older people and also who have a better understanding within the economic conditions because they've studied economics but on the other hand you have much older people who have been trading for donkeys years and dedicate their lives to this so therefor they have the time to research about the economy and also have a better understanding on technical analysis. In regards to a 14 year old becoming successful is never impossible but unlikely because as you said the maturity is nowhere near substantial enough. But the conclusion for this discussion for me is that the younger you start the better of course with more experience you'll gain a lot of knowledge, but trading is all about YOU and THE COMPUTER no one else. Some people learn quicker than others, WE ARE ALL DIFFERENT. It doesn't take a genius to learn about the economy if you put in the time to study it.