Certainly tooth decay existed in prehistory.
Here are a couple of Mayan Sculptures, I presume from at least 500 years ago, perhaps much older.
I can't tell if they are two depictions of the same figure, or different figures. Obviously depictions of bad teeth in sculptures is not indicative of the overall prevalence of tooth decay, but it is at least an indication that it occurred, especially in the elderly.
Many ancient skulls appear to be missing teeth, but without knowing more about both the skulls and dentistry, it would be hard to tell if the teeth were lost before or after death.