Dead by old age

Of course you can have aging on only for the active household. So you don't have to worry about all the other of your sims aging. As someone who has played with those settings, I can confirm it does work.

Now how could you go with old age death? Well, here are some suggestions I have:

• Play with aging on until your elder's aging bar is near full, and then turn it off. You may turn it on again once you are ready to let the sim go.
  
• Use potion of youth from the rewards store so other sims won't age up.
  
• Separate elders into household of their own so you can have aging on without having to worry about other sims aging up.
  
• If you have Paranormal stuff pack, send elders to haunted homes at night. Each time your sim gives their soul to spirits, their aging bar will fill up considerably. This way, you could have the aging bar fill up fast.
  
• If you have mods that enables all cheats, you could just use sims.age_up cheat. Though I really want them to add a cheat to easily age up sims or pets with a patch...

I hope these are the tips you were looking for.

About how many time will your sim have left after the bar fills up, I unfortunately don't know the up-to-date info about that. Though I do know that at some point, it took 12 hours on short, 1 day on normal, and about 4 days on long lifespan for elder to die after their aging bar reaches full. However, this time will be extended when your sim has either Active and/or Long Lived traits.