As a gamer, I have to say I've played M rated games before I was 17, in fact, I played Mortal Kombat (rated M) when I was about 12. The game did not turn me into a crazed maniac. Since these guys were just trying to buy an M rated game and they looked to be almost 18 anyway, it's really not that big of a deal.
That being said, you are not their parent or guardian. As much as I hate to choose this option, that was a bad move on your part. You have no idea how their parents are going to react and since they are not 17 you are not in a position to make that decision for them. Like I said, I hate to have to choose this one, but I think I'm right.