They use tooth wear for the most part and on very young deer if all they have is the jaw bone, which teeth have erupted. However, it is a little more complicated than just the tooth wear and the amount of dentin exposed, they would have to know where the deer came from, sandy soil erodes the teeth faster than clay or loamy soil. I used to have a chart that gave examples of wear and age with pictures of jawbones, I no longer have it. Each year in PA, teams of biologists and biology techs go around to cooperating butcher shops and age the deer heads before they are sent to the rendering plant to get a pretty good idea of the make up of the herd ages. I haven't tried but I would bet you could find such a chart on google find.