You may, or may not, have heard about an article published in The Inquirer that states New York State will prohibit the sale of videogames to anyone 30 and under. The Inquirer is surely not the bastion of quality journalism, so if you found the report to be too outrageous — it is. The correction to the article is that New York State is currently in the process of examining a proposal that wants to make sales of rated-M games, similar to the selling of alcohol in New York. That is, if you don't look over 30, you will be carded before your purchase is made. So the same way that a convenience store, or restaurant will card you when you purchase (or attempt to) an alcoholic drink, is the same way that a clerk will have to.
So don't fret, if you're under age you'll still be able to buy your rated E and T games. But if you're going to buy a rated-M game, you'd better have some ID on you. Pending, of course, on if the bill gets passed or not. But personally, being a New York City/State resident, this law will probably go largely ignored by many retailers.
Bad enough I rarely get carded when I purchase drinks anywhere I go (I'm only 22, mind you), but I highly doubt a cashier at Best Buy will ask me for ID before I buy a copy of Grand Theft Auto IV. Unless the penalties for not doing so are heavy, retail employees in New York will probably scoff at this.