Just happened to me and I was watching him. Dad was standing in yard with daughter. Aging off. He has rewards immune to heat/cold etc. Perfectly sunny day. Just talking to a neighbor holding his toddler and then poof, no Reaper just turned into a tombstone right on the lawn. I immediately exited and did not save. I did however look up his Sim Info when I went back into the game.
I've been playing with this sim for a LONG time with aging off because I'm doing a special challenge and I've used the character before in other games and pulled him to this one.
The info on him when I went back in to the save from a couple simdays before said Age: Young Adult (1.4%) Current Age Span 452 days 448 + delay 4 Aging/AgeUp/Death:Disable/Disable/Enable
I'm thinking that last Enable might be the culprit? I did cheat the trait immortality onto him to see if that works or not. I will have to play a couple more sim days to see if it happens again. I might try deaging and then reaging to see if that works if it happens a a second time but I feel like he reached the end of the span and the game 'culled' him.