It happened on my legacy lot - Addison Cross, 3rd generation heir Just. Would. Not. Die. I couldn't cowplant him. Rodney's Death Creator did nothing - anything selected would just drop out of queue. I aged him down to adult, then back up to elder, to see if that would clear up whatever happened to make him not die.
In frustration, I moved him into a small house at the back end of Downtown. I planned on playing the lot until 5 am the next day, as is my custom, then just leaving him there. I'd figure out the story angle later, lol!
At 10 am, Grimmy showed up and Addison shuffled off this mortal coil, just like he was supposed to. I have yet to figure out why, but in all my searching, no one else seems to have reported this, so I offer up my experience as a solution.
As an added note, I ran Mootilda's Hoodchecker, and it reported nothing about Addison except to move his grave to my designated cemetery.

Welcome to MTS, Noir Moon!

Happy to hear you were able to get your sim to meet the Reaper! It's also great that you made a point of searching hard for a solution before you tried moving out and in somewhere else; there is a lot of accumulated knowledge out there, and a lot of it is indeed able to be found in a search.

If you come across a similarly difficult problem in the future, though---believe it or not, once you've searched and found nothing, that's often the best time to come to this forum and ask the members for their thoughts. I've often been pretty impressed at how someone's description of their seemingly impossible Help issue brings out another member's (or several others'!) hidden-away knowledge---which turns out to be exactly what is needed to solve the problem.

Moving a household out and then in (to the same home or a different one), which seems to be what led you to mark your thread Solved from the get-go, does work to resolve nearly all problems that aren't mod conflicts, because Maxis designed that process to reset so many things that can go wrong with lots and sims.

However...you may not have been with us long enough to know, but that's definitely in the direction of what many here might consider to be nuclear options! So there are many tricks that folks here know, and collectively all those make it uncommon that someone we're trying to help will need to resort to just moving out/in. (For example, though I'm not one of them, there's folks here who can read error logs like the backs of their hands---and give you straightforward fixes for what's revealed to be going wrong.)

So your wish to add to our knowledge is definitely appreciated! But I also hope you'll give us a chance to work on your issue along with you the next time around.

Looking forward to helping you---and to seeing more of you around here.

Also, something else interesting to note - Addison and his wife had full life bars on the same day. Maybe the Grim Reaper Union has made a ruling that Grimmy can only come once a day for old age deaths? LOL
Error logs are supposed to be written in English, but darned if I could ever make heads or tails of them.
Another reason I posted this is when I did my initial search (on my phone), all sorts of answers popped up - for Sims 3 and 4, which of course are entirely different kettles of fish!

Absolutely they are different. But that's just the way search engines work---they'll always want to show you the most recent popular results. Since TS3 and TS4 are newer, that's what you'll get without some of deep searching that folks around here can help you with.

No, Grimmy can come as often as necessary in a day for old age---usually one and then the other.

There are folks here who can teach you to read those error logs! So next time you run into trouble, feel free to upload one.