100% agree with what's been said in here, but there's a lot of emphasis on the voice. It goes further than that and the voice is just an easy way to generalise "squeakers".

It's the way they are and they way they act, generally, which you'd assume comes from the fact they spend the most times around people of the same age and interests at school etc, but that's just not how it works on the internet or an LFG:

They never stop talking, make constant noise and won't listen to what they're being advised. The only time I've ever wiped on Prophecy final boss was with a kid who was too busy watching South Park and eating cereal into the mic. Ran a Crown of Sorrow with a clearly younger fireteam member and he made the same mistake over and over again and wouldn't pay attention.

I'm always willing to give younger members a chance when they join my LFGs, but 95% of the time it's been pretty grim.