Age has plenty to do with it....father time is father time. Carries or not, father time still comes into play.
WIlliams is fine in the since he could techincally play for a couple more years. Issue is he should be a near vet minimiun RB for those years and .
Age and carries both factory in to decline in production but I think we can all agree that D-will and Stewart very little in game wear. Stewart has never had more than 215 carries in a season and williams has only had 200 plus carries 3 times in his career. Look at a back like Lynch who has rushed for 200 plus carries over 5 times in 8 seasons, he is a back I would expect to fall off a cliff.