My little guy is 4.
He proudly takes on the role of attorney-general in our house.
This really wasn't that hard, since he asked me the question directly; "What games can kids play and what ones are for grownups?"
So, since he can't read yet, I've taught him that if there's a game he likes the look of, to look for "what colour the game is". And he knows to look for the rating sticker.
If it's Green, (G) he can play. If it's yellow (PG), he can probably play, but he's gotta ask if it's OK.
If it's Blue or Red, then no. He's simply not allowed to play it, and neither are his siblings. The only exception is Burnout Paradise. It has an M rating, but since there are no "people" in it, there's no danger (imho) of him seeing anything violent enough to cause him permanent harm.
FWIW, my 15yo rejoiced that he was now "old enough" to play games like GTA that have an MA rating. (and since he now lives with his mother, there's not much I could do to prevent it, but that's a whole other story). Up until then, I didn't even permit him to watch me play, because of the adult content.
My other kids have sat and watched me play the likes of MAG, but I usually play :
(a) after their bedtime, and
(b) with headphones on so they can't hear the language
Back on topic: I was in a game of MAG a few weeks ago, same story. The squad leader was 12. He was OK as far as playing and K/DR goes, but he had no grasp of tactics or what objectives to set as FRAGO.
I was also in a MAG game with a squad of other TOGgers, and we ended up with a kid in the squad... until he had to drop out mid-game to go and have his dinner, or do chores or something.