Actually, Everyone is nontheistic (For the first few years of life).

The "does not believe in god" definition is not atheism, it's nontheism. Somebody who has no belief on the existance of God whatsoever.

If you look at the true definition of atheist then you'll see it's impossible for an infant to think that way.

/me concurs with Wesc. An atheist is one who denies the existence of God. Someone who is ignorant of the possiblity of God cannot possibly deny his/her/its existence. Thus, infants, children, people who've lived in a jungle somewhere all their lives, these people are nontheists. Someone who has had the concept of God explained to them and rejects it, those are atheists.