Kids won't understand the world around them, their culture, history, politics, and so on if they are ignorant about religion. Western culture is incomprehensible apart from knowledge of the Abrahamic faiths, in particular, Christianity.
At what age should children be introduced to atheism? After they understand their roots, which aren't atheistic. Atheism benefits no one, as far as being informed of the world around them goes.
Ignorance of Christianity is the formula for ignorant human beings. And this applies to self-professed "Christians" as well.