I always churn out the same thing on these threads but here goes:
My background is games retail. It is likely that most of his friends are playing the 16/15+ games and I would wager that a large percentage of them are also playing the 18+ ones as well. Experience in talking to parents over the counter tells me that a lot of parents:
-don't give a shit
-think that games are not as influential as television/films
-give in for an easy life
-are not aware in the slightest of games ratings
This would accout for the majority of the people I spoke to over the course of my career.
Of course, that is not everyone.
My opinion, as a parent, a gamer, and an experienced games retailer (and therefore reviewer of games of many classifications)? Let him play the games suitable for his age. If you are unsure, have a go yourself, or ask in a games shop for a review, or their opinion, or check online. I would not let an 11 year old play 16+ games, no matter what his friends were doing, because in a year or so, it's going to be 18+ that he wants to play, and what are you going to say then?