If you just talk to them, they will listen.
I think you're arguing with the way to do it, not the concept. The basic idea of a lower age limit is sound. Kids don't know how to handle certain topics well, even when they think they can. As an example, I don't want my 7 year old to watch the news because she has nightmares afterwards (depends on the news, of course, but I have no control over that). It's well known that news causes people to feel bad, but adults can make their own decision about this. She's my kid, and even though she may be smart and interested, it's my judgement call that she'd be happier if she doesn't watch that. And I can and do explain to her why. If she wants, I can let her watch kids news or let her satisfy her curiosity another way.
I don't take to the idea of lying to children, inventing fantasies to help them cope. I'd rather that my kids know what I'm thinking and what's happening. But I do think that not letting them know the full details or trying to make sure they don't experience some things is part of my job.
Even if I didn't know about games, I think it's a valid thing to say to a kid: "According to the rating of this game it's not for your age. People use language in a way that I don't want you to pick up, it has graphic violence which I feel is too much, and other stuff that I'd rather that you don't experience." I imagine that a teenager won't listen to me, because they tend to think they're smarter than they are and that their parents are dumber than they are, but I still think it's a valid thing for me to decide.
Edit: A general note about the reasoning:
A parent's decisions regarding what the kids should and shouldn't do largely come from the desire to have the kids grow up into happy, contributing adults. Each parent has his or her own way to define this and deal with it, and it's certainly possible to contest a certain way, but it still comes from caring about the child.
A child's decisions largely come from the egotistical reason of "I want it". So when a parent limits what the kid can play, the kid can contest that "it's not that bad" but can't really argue that there's a good reason they'd play the game. The kid's sole argument it that he or she wants the game.
So basically, the claim that kids are smart is wrong (in this context). They're clever about getting their way, but since "their way" is an egotistical thing that they want, the end result is rather dumb.