I think a lot of it has to do with how much of an outside influence a younger person is getting besides the video game itself. If they've got a dad, uncle, friend, etc. who's taking them out and properly showing them shooting sports in between rounds of Call of Duty, I think they've got a much healthier chance of getting the right idea than if their info is coming from video games, TV, and the playground.
I'm a rather avid player of CoD myself, but I'd had family members show me the ropes of firearms since I was 10; long before I was really into shooter video games. I'd like to think I wouldn't be one of these uninformed kids that thinks an M4 behaves like it does in CoD, but I'm also glad I don't need to find out
The gun is another part of our culture that has a lot of myth wrapped around it. It's no different than the way kids view a car, or anything they can't yet get their hands on. Pile in on top of it trying to pass off as 'cool' in front of their friends and it's small wonder they hold a gun sideways their first time if they're uninitiated.
On a personal note, I think what irks me the most is the question, 'How do you think this gun would do in a zombie invasion?' I don't mind hearing it in jest or discussing it for the sake of amusement. But when some teen asks with a straight face, and wants to know how well it could kill a zombie, it's another one of those questions that just sounds they've been playing too many video games.