I was playing Doom II when I was 11.
The only real complaint I'd have is that the kid will undoubtedly be shitting up online play by screaming into the mic.
Well, not necessarily. I imagine that if any significant percentage of kids did things like scream into the mic during online play, it would be far more unbearable than it is now. Chances are he's going to be perfectly harmless, and that often times it'll be a late teenager or adult being the obnoxious one in a room.
Come on. We were all kids once. If, say, Doom II had the same online capabilities, would you have been the kid annoying everyone back in the day? I think the default assumption that an additional kid playing = more annoyance reflects that we're getting old more than anything else.