How long did your Doberman live and what were their weight? My 100lb doberman (not fat) is aging well beyond his 8 years

My 100lb doberman (not fat) is aging well beyond his 8 years. He has a strange issue of sometimes when he gets up his legs are all wonky. Had every test in the book and no one can find an issue .. so wondering if its old age? And I am wondering if his larger size has something to do with it ???