Do you use any mods? If you're using MCCC & flagged the sim as immortal (& forgot), that would probably do it.
I don't remember if there are any options in the game settings that could be relevant, but it might be worth checking. If you have the longest sim life spans, for example, it may take longer than usual.
Good luck! and farewell to your sparkling sim, whenever his time comes. 😉
eta: Oho! There's a thread further down about this, with an EA response, too! Apparently it can take a long time for 'em to die on long lifespan settings.
If you enjoy reading threads about this, there's still another one just a little further down. Hmmmmmmm, that's three threads on the topic; wonder if it's taking longer than usual since the patch???