I'm of the opinion this ought to be left up to parental discretion, not the legislators. Unless parental discretion is failing on all fronts and it is actually causing adverse side effects, I don't see what the big deal is.
However, I have always wondered if over time a counterargument would be developed against that. That is to say that parental discretion is no longer viable somehow. This would need specific proof in several areas to support it.
I.E. Responsibility and trustworthiness, initiative and work ethic, school performance, general behavior and ethics, and obviously violent tendencies and attitudes.
While I suppose correlations could be made I have not seen much to suggest playing video games necessarily causes deficiencies in the areas I mentioned. Some kind of saying "Correlation isn't the same as Causation".
For fitness and obesity it definitely factors in more greatly here because obviously you're not doing much physical activity.
Hey, stores make policies of their own as well. I.E. K-Mart won't sell "M" rated games to minors at their discretion. I think Radio Shack is similar--hell they don't even sell laser pointers to anyone under 19, either. If I am not mistaken, even Hillary Clinton is big on heavier regulations for video games.