I am 21 and an A320 F/O. You will find many 21 year old pilots out there, some more mature than others. Clearly, it takes a certain amount of maturity to work in this industry. The company I work for doesn't operate any other type, however, if I was to be in the place of some that work for companies that have a more diverse fleet, I wouldn't feel it to be at all unreasonable that I could have the opportunity to fly a large widebody in the next few years.
That said, I started training at the age of 19, and some of my "classmates" really had no maturity at all, and it was pretty clear they didn't understand what was required from them by the industry they were getting into. Those guys really struggled through their training, some even dropped out. This is not necceseraly due to low aptitude, but more related to not knowing the difference between time to work and time to play.
Everyone is different, I believe that you need to put in 150% to get 100% out. Work hard, play hard, and you can achive anything
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