Somebody who has orthodontic problems, especially teeth coming in slowly.
"My orthodontist said I'm dentally advanced."
"Mine said I"m dentally retarded."
Someone who *knows* they need to go to the dentist but keeps putting it off because they are scared, they don't want to spend the money, or they think the problem will just go away on its own.
Bob: My teeth are killing me.
Steph: Why don't you go see the dentist? You're always complaining about your teeth.
Bob: I don't wanna go.
Steph: Man, you are dentally retarded.