1. In CAS, create your Teen.
2. Add Sim, and make their age Elderly.
3. Pick any Aspiration (doesn't matter). Pick the following Traits: Outgoing, Neat, Active. (Probably doesn't matter much, but will speed up Need decay)
4. Move your new family into a lot, immediately pause and build a 2x2 square room in the backyard. Grab a door from inside the main house and put it in this room's wall.
5. Have your elderly SIm enter the room. Go into build mode and put the door back where you got it from.
6. Play the game, working on your Teen Sim's homework, meeting people in front of your house, buy books...whatever - just DO NOT travel anywhere.
7. After 2 or 3 days (tops), you will see in the Needs panel that the Elderly Sim's HUnger, Energy, Hygeine, and Social are dark red.
8. Select your Elderly Sim. Open the Console by pressing CTRL+SHIFT+C. Copy/paste this in the Console input window in the upper-left of your screen:

sims.add_buff buff_Death_ElderExhaustion_Warning

Press Escape to close the Console. Your Elderly SIm should die within a few seconds to a few minutes or hours. (mine died instantly)

Note: If you travel off the Lot, your Elderly Sim will be removed from the prison room and placed back in the main house.

And NEVER EVER use the cas.fulleditmode cheat to change the teen in CAS. You won't be able to go back, since there is no adult in the household.

Source: https://twitter.com/crinrict/status/510047874378649600

thx this really helped

Actually, you don't have to grind down the filler-elder's needs. Applying the killer moodlet should strike them down instantly.

This even works with my "no elder exhaustion" mods, because those prevent the moodlet from appearing through normal gameplay. You can still force-add it.

Can confirm. Be sure to have the elder actually selected when you enter that cheat code, or else the selected Sim will die instead. Not that I'd know from experience or anything. :x

Huh? Last time I forced the elder exhaustion moodlet on somebody not elderly, they weren't killed...

Last time? My, my.

Don't use mirrors or wardrobes either - you can go back by pressing "X", but you can't save your changes to the no-adult household anyway, so it's kinda pointless.