The two associated Hyaenodon horridus mandibles from Nebraska puzzled me because they appear to be from an adult but when I removed some matrix i saw gaps for a molar and a tip of a tooth peeping out. I looked into it and it seems that the last molar doesn't reupt until age 3-4 so that is roughly the age of this animal. To verify, I asked a good mate to xray it. Th huge erupting molars are clearly see in the imagine. You can also clearly see the tooth roots below the crowns of the teeth and below the gum line (ie embedded into the bone). You can also clearly make out the mandibular canal (long dark line running ventrally below) the teeth - it's where the major veins, arteries and nerves of the bone run. Great view of millions of years old bone and teeth! Confirms that the teeth are erupting molars and gives me a great idea of the age. Not, quite juvenile, more like subadult.