Personally i would define it as the following: -
Squeaker
1. Someone on voice chat that has not hit puberty or has an abnormally high-pitched voice. They will often use a ridiculous amount of profanity to try and offset their lack of audible maturity.
2. A (suspected) young person on open channel who is clearly clueless yet feels the need to rant continually on a given subject, often with the use of capitals or very poor spelling, which they know nothing about.
3. A clueless (suspected young) noob who feels the need to mouth off at you once you shoot them down.
4. Term which can be used for any young person(s) in game.
Basically, i would define it as any person i suspect of being young and clueless a squeaker in game.
e.g. "Megadud stop squeaking".
But thats just me.