I think you should send an email to them explaining the situation and how mortifying it was. That's very unprofessional of them. Even if you did have some kind of mental disability, that's so so rude of them to talk within earshot of you. If someone is independent enough to go to the dentist without being chaperoned, they deserve to be treated like an adult. I'm so sorry that you experienced that. That entire office should be the mortified ones, not you.