Graphite, dirty engine oil, wood stove soot, many sources of dirt and grime to rub in and make look old. Small can of Minwax 'true black' or 'ebony' (ebony is less black than true black) applied lightly and wiped off will gray the wood.
Are you through the 'fuller plast' into wood? or just chipped off the color coat? If you are not down to wood you won't get any wood aging tricks to work out.
Lol, steel wool. So many players suggest using it (typically to clean fretboards) and it is really the devils tool around guitars. The wool bits break off while using them and get attracted to magnets very easily.