Well, in the prose story Lessons in Passing
, Mantricker is having trouble not thinking of Bearclaw as a cub anymore. He muses that Bearclaw is physically mature, but still not mentally mature, and he hopes to stay alive until Bearclaw is fully mature. Of course, he dies then. So I would imagine that Bearclaw was probably 20-30, kind of like Cutter when he became chief.
It also describes Mantricker as a young chief at this time, so I think his reign must have been really short, maybe in the 100-200 year range.