Train up a Child

Someone quoted the verse to me today, the one in Proverbs where it says, "Train up a child in the way he should go and when he is old he will not depart from it." Prov. 22:6 It was in reference to a guy who had became a Christian as a teen and then fell away from the Lord and now has recently come back to the Lord. I am excited for this guy. I think that is great! But my question is this: Why does everyone assume from this verse that it means the child will fall away and then come back to the Lord? It doesn't say he ever fell away, it says he will not depart from it. So to me this verse means train up a child in the way he should go and he will never depart from it. Hmmmmmmm.....................