I agree, up to about junior high you can restrict smartphone/tablet usage. At about 12 it really seems like the social pressure to have a phone becomes really intense, unless there's some school wide restriction.

I guess one question is how you introduce it in a way to inoculate them against the addiction without just allowing it. Mostly, I've just seen temporal restriction, but by 16 or so they can afford to have a "burner" so it's much harder to restrict at all.