Adults should be allowed to play what they want. However...
As much as I love to play violent video games at my age, I applaud any effort to keep them out of the hands kids and teenagers. Specially younger kids. Its much worse than violence in movies and if anyone cares to disagree I will post my arguments later.
Unfortunately, a big part of the problem is there are too many parents who fall into the category of being too ignorant, too irresponsible or plain incompetent.
I am disgusted that when I play COD online there are many kids playing who shouldn't be. Shame on the parents for letting them or for being so ignorant about policing their kids Live accounts etc.. and not setting up the "family" settings. I have even played against some dumba$$ who let their 4 year old kid and jump in for some games.
I also remember downloading the demo for Saints Row off Xbox live. When playing it, I could not believe the demo had so much vulgar language and excessive violence a la GTA. I loved it myself but I could not help thinking how many kids could easily download this demo if their parents weren't privy to the Family settings etc.