Sneak out, go buy the game, install it and hide the packaging somewhere safe?
If your parent's ask for the packaging you can always tell them you lost it.

I don't get why parents make such a big deal out of this, it are videogames, it's not like you're gonna go shoot someone in real life because you do it in videogames :s.
That's one of the good things of my parents, they think that videogames are a good way to loose agression. They gave me GTA when I was 10, and they've never cared for the Rating of games, "everybody will be confronted with violence, curses or sex sooner or later anyway".

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