Actually, most peoples teeth are worse today than in roman times. It's quite funny.. you can see ancient skulls with quite healthy teeth in them, right up until the introduction of refined sugar... at which point, everyone's teeth started falling out. No sugar in your diet, and your chompers will be perfectly happy for a very very long time.
Also... (Americans in particular, I'm looking at you here) teenage bracers and Game Show Host bleached white is not the same as healthy. Pretty? Sure. Healthy? Not necessarily.. and if you're chemically whitening them, that's not a good thing for them.
I would have guessed starchy foods because they get stuck in your teeth. Sugar just dissolves.
And coca cola