I've played with all sorts of ages, from 10 to 45. There are skilled and unskilled players in all ages. Granted, most skilled players fall in the ages between 17-26. I've played over 300 matches on XBL and I've yet to be betrayed on purpose (maybe I'm lucky). Sure, my teammates will throw a grenade in the middle of my head-to-head match with the other team every once in a while, but their just trying to help out. If they're being jackasses and not helping the team, I'll betray them.
To avoid noobs, just team up with some good people for a couple of days and get past level 15. However, you better be able to hold your own, or you'll see yourself back in the "kiddie" ranks.