I know at least one kid who lied about his age and refused to talk on vent because he was 13 at the time. He was ashamed of his age and his voice even though (as far as we could tell) he acted relatively mature at the time.
Of course until that incident the mature thing would be to tell us that he can't speak on vent rather than try to hide it (he said that he had a vocal chord problem and couldn't talk well). Eventually he came clean about it and everyone forgave him but it's kind of illustrative that even the most well-meaning kids have errors in judgement.
As for MMORPGs, the kids tend to get either weeded out through an application process (ex, they get you to talk on vent for an interview) or they get weeded out through culture over time (ex: if they find out the age you get ribbed until you want to quit). In almost all of the hardcore raiding guilds I've been in in WoW, all of them had an age limit and the youngest person they recruited was 17, and he was considered the baby of the group. He was a good player but when the 17-year old is considered young you know that there'd be no tolerance for 12-year olds.
Why not? Well in a game like WoW where you are expected to be online for the block of raid time (reminder, this is a hardcore raiding guild we're talking about, probably top 500 US guilds) and kids tend to interrupt with that. They have to "go do chores" or "go do homework" or "go to bed". Their parents are asking them to go to do whatever here and there. It's disruptive and difficult to work around when everyone else is able to block off time. Now if one of these things does happen, the guild would just replace his raid spot. A mature kid will handle it. An immature kid will whine about how his raid spot is given up (note that this happens to immature adults as well).
If the adults are playing a game when they are in a mood to teach people (like a casual WoW guild for example) then young kids are tolerated much more. In fact if you're in a good MMORPG guild and talk with adults a lot as a kid, it is actually going to affect you and you will end up acting more like them. Most of the time 12-15 year olds don't get a chance to hang out with adults due to school, so those who do get the chance to listen to what adults talk about and how they handle situations get the opportunity to learn social conventions much faster. Note that this means that you have to play the same game with the same group of people frequently and form friendships, of course.