[Keeper of Dol Guldur]

I don't think he would have had to be that old, alltogether. In the Hobbit it stated that after the Dwarrow-Goblic War (I really have to stop terming this stuff), for the Mines of Moria, most of the goblins were wiped out, and they had been secretly breeding in the Misty Mountains only in recent years.(which is still a long time) So he didn't need to be ages old, but the topic "Of Beorn," which was started by none other than me, the Keeper, not all too long ago, has some rather keen insights towards his powers.