If I recall correctly, this is because American retail stores like Walmart and GameStop will not sell games where you can hurt or kill children, meaning almost no games do it. So most kids in games are basically invincible.

This is a bit of a tangent, but I do think this is why they make a point to say that Ashley in Resident Evil 4 is twenty. Being able to accidentally kill her in the game is a big deal, so I think they wanted to make it clear that she's an adult, even though she seems a bit more like a high schooler, at least to me. Granted the sexualization of her was probably as big of an influence on them making a point that she's over eighteen.