We are talking about the USA here. Excessive violence is of no concern for the vast majority of parents, so kids won't have to do any convincing.
Besides, ratings for video games are merely guidelines, not legally binding like the sale of alcohol. Most clerks aren't going to care if a kid buys an M-rated game on their own.
Funny you should say that. I remember back in 2009, the day after I bought my first PS3, I was at Best Buy to buy Grand Theft Auto IV. I was 18 by then, but the clerk still asked me for an ID. I'm sure several major retailers still ask for ID to minors. However, Gamestop sometimes didn't ask me for any ID.