|Fig. 3 (Predicted ages are derived from human radiographic
Neanderthals, but not ancient H. sapiens, clearly deviate from the developmental pattern of our species, with teeth (and hence most likely all them) developing faster than among us.
It seems from this data that a 12 y.o. Neanderthal would be as developed as a 16 y.o. modern human. One can easily infer that they should reach puberty some 2-3 years earlier than we do, though this is not directly in the teeth and also shows some notable variability among modern humans.