I just wanted to add some info on this from my own experience. Hopefully it is helpful for someone else.
My sim died from old age. She had accomplished all the primary goals that I had for her, and was ready to leave this world to give place for the heirs. But, this was apparantly "not her time", she was saved by one of her dogs. But unfortunatley she didn't return to a life quite as good as the one she had left:
1, She was no longer married to her life long husband (showed up as ex wife/ex husband). This ment she had to spend her final nights sleeping in a separate bed from him...
2, She lost her employment. I just had time to think "Well, at least now I can try to max her carieer before she dies next time." Well, nope... Had to reemploy, and ended up on rank 4 instead of 8 where she was before.
3, She lost her moodlets, including "Very fulfilled" from completed LTW, which was supposed to last for several more days.
4, She lost her choosen wishes, including "Become worth 1 000 000" which would have rewarded ~20 000 points if completed.
I guess these would all be neglectible setbacks if it was a sim that I really wanted to keep alive that was killed by accident, but for this old lady it felt kind of sad, having to live her last time a shadow of what she once were...
And, additional info for those who wondered: Yes, the sim will die from age again at a later occation. At first I thought she might live forever, but that was not the case. It took a while, her husband died before her, but then she finally passed away.