As a 17 year old, I tend to disagree to this.
First of all, broad generalization of groups of people isn't the right way to critique, which on that part I agree.
However, the generalization of the maturity of kids in the tf2 community hasn't changed for me. I have not met a single kid that has changed this viewpoint for me. Every >16 player I have ever met in a valve server, community pub, or mvm have been whiny, screamy, immature, and toxic. "Guys guys pls dont kil me Im frinedly i hav "x" on and "x" on to show im friendly pls no kill." "Exotic Muselk meme" "le lenny face out of context" "How do i do "x" as "x.""
I may have started playing the game when I was 11, but that still doesn't mean I was mature. Hell, I even asked for free weapons when I new they were a rarity back then. I even used Drunken F00l's idler because I was so in a hurry to get items. Hell, my account got stolen because I wanted a free Minecraft account. I was the epitome of TF2 "autism" and I still cringe thinking back to those days.
Here comes 2012. My 14th year of life. I'm a brony, a memer, a goddamn kid who watches minecraft LP'ers on my Ipad at restraunts. I still was immature. I still squeaked and squawked onto my microphone while playing tf2. 2012 is also the same year when I started playing tf2 again. It was the same year I found my first weapon after the mannconomy update: the Overdose. It was the same year that I tried to heal a heavy by firing needles at him. It was the same year I matured. The same year I finally found out that all of the things I've been doing represented a child. A child that shouldn't be playing a M-17 game like tf2. A child who shouldn't be annoyed by people who call him "annoying" for talking into the mic. My new years resolution for 2013 was to mature, act my age, and grow a fucking pair.
My conclusion? Kids in M-17 games are cancer, whether they're mature or not.