Best advice I could give is to watch casts/stvs and get a feel for the role you want to play, rollouts are essential, and I would reccomend just using newbie mixs, because lobbies tend to be a joke 80% of the time, or just leaky on 24/7 sniper.
Age doesn't really matter, if you can keep your cool and play well then people will respect you. There are douchebags out there that will discredit you for your voice, just igonre them, feeding the trolls isn't a good idea.
Finding a mentor is a great idea, reviewing demos has really helped me see my mistakes. DM is an important factor, but you could be really good in mge and not do well ingame due to just positioning or another factor. The best way to practice is to just play the game.