What age do you think is too old for a backless booster seat. My son is 14 and still uses the same plastic booster he has had since he was 6. That's his choice though.?

He's way to old to use a booster seat, tell him he can't use his baby seat any more and throw it out. If he protests smash the stupid plastic seat so he can't use it. He's 14 you should have done this when he was 4