This is a cool idea. Why not both? There could be one trait that gets added automatically, based on how the sim lived, ANDa few reward traits that were only unlocked for elders? That doesn't seem to hard, considering all the traits that got added for parenthood, and if it's based on mood or something stats already track, you wouldn't even need to add a whole "wheel" thing like they did with that.
I mean, I also wish graves and urns were different depending on household money and points earned at time of death, but this would make the elder stage feel a lot more earned and vibrant.