By adult I mean over 18-20 years of age and with adult appearances. Nothing against anime with younger characters, but interested in finding some with older characters.
In no particular order these come to mind. Most of these are outside what I typically watch, but I did enjoy them.
Monster (Crime Drama) https://anilist.co/anime/19/Monster/
Mushishi (Psychological Slice of Life) https://anilist.co/anime/457/Mushishi/
One Punch Man (Comedy Action) https://anilist.co/anime/21087/One-Punch-Man/
TRIGUN (Action Adventure) https://anilist.co/anime/6/TRIGUN/
Hellsing (Vampire Action) https://anilist.co/anime/270/HELLSING/
Overlord (Evil Fantasy) https://anilist.co/anime/20832/Overlord/
I think a lot of the other answers here are being too generous with your rules and not giving you characters that both look older and are older.
Overlord has Mare and Aura though? Who, ignoring numerical age, have the appearance of a person under 20.
And Victim is a literal fetus, though Victim is only in the show for about 3 frames.