How old do you need to be to create a video game?

It might help to be old enough to read, but it probably isn't necessary.

Seriously--how old do you have to be to paint a picture, compose a song, write a story? There's no age limit on creativity (in either direction) or the use of the tools.

As for marketing and selling a game, that's a different matter. You might have to be able to enter into a contract or have an adult representative who can do it for you.