It sorta depends. Is the kid more intelligent than most? Can s/he separate reality from fiction? Is s/he likely to pick up an actual gun and see if it really does the things s/he sees it do on the TV screen?
For the most part, no, parents shouldn't let kids see that stuff. Because kids are retarded, and they learn by two things: Example and experience. They will NOT learn ANYTHING told to them. Such is the workings of humanity's collective idiotic mind.
Also, those ratings are there to help parents discern whether their kids are "mature" enough to handle what they're seeing.
If the kid in question is reasonably intelligent and can separate reality from etc, then more power to him/her. I myself was playing Vigilante-8, a T-rated game, at age 7, and saw GTA for the first time at age 10. But since I could gather that doing stuff like welding a machine gun to a car and blowing stuff up is a bad idea IRL, I didn't do it.
Sad part is, soooooo many children want nothing more than to do just that, and thus should be sectioned, and failing that, kept away from higher-rated stuff.