Robert Quinn has the most career sacks by a 31-year-old player, with 18.5 sacks.

most sacks by nfl player at age 31
Interpreted as:
most defensive sacks by NFL player at age 31
QuinnBabinPorterDeanCampbellRiceFletcherPryceStrahanMartinDouglasO'NealGreeneWhiteWhiteTaylorThomasAllenArmstrongJonesHamptonWalden18.518.017.517.514.514.013.513.013.013.012.512.512.512.512.512.012.011.511.511.511.511.0
NAMEGPSCKTKLASTCOMBSCKYINTINT YDSINT TDFFFRFR TD
Robert QuinnRobert Quinn1618.5381149133.5000400
Jason BabinJason Babin1618.035540107000300
Joey PorterJoey Porter1617.537114896000410
Fred DeanFred Dean1617.50000
Calais CampbellCalais Campbell1614.547206796000311
Simeon RiceSimeon Rice1514.03374089160600
Simon FletcherSimon Fletcher1613.50000
Trevor PryceTrevor Pryce1613.0371047113.5000200
Michael StrahanMichael Strahan1613.055157076000500
Wayne MartinWayne Martin1613.00000
Hugh DouglasHugh Douglas1612.54585389000200
Leslie O'NealLeslie O'Neal1612.50000
Kevin GreeneKevin Greene1612.50000
Reggie WhiteReggie White1512.50000
Randy WhiteRandy White1612.50000
Jason TaylorJason Taylor1612.052217380000421
Derrick ThomasDerrick Thomas1512.00001
Jared AllenJared Allen1611.533195274000200
Trace ArmstrongTrace Armstrong1611.50000
Sean JonesSean Jones1811.50000
Dan HamptonDan Hampton1511.50000
Erik WaldenErik Walden1611.031114264000300
Jason HatcherJason Hatcher1511.03474192000200
Julius PeppersJulius Peppers1611.03343760000320
Lance JohnstoneLance Johnstone1611.02563166000500
StatMuse has game-level data for sacks going back to the 1982 season.