I use MCCC's Mark for Death (and from there I select "Old Age" for the cause) for elders I don't play. A completely peaceful (and touching, if you happen to witness it) death happens within 24 hours, usually pretty soon after using the command. For any elders in my currently active household, I do the "Kill Sim" command with the same cause of death. Edit: That one happens instantly. The animation is the same for both, peaceful, non-violent, and quite nice... as death goes.