I've never seen a elder imaginary friend. And I play on full autonomy as I find that hilarious more often than annoying so I have no idea what's going on. I also have my mods set to ensure every baby born in my game gets an imaginary friend so I have lots of them around. Imaginary Friends don't count towards the household members but they can have relationships with any sim including their owner. I've had several marry and have families. Their children can inherit the hidden pyromaniac trait. Or be born as Imaginary Friends that also have their own imaginary friend in their inventory.
Edit. I need to clarify. IFs don't count as part of the household until they are made real. Once they are real they are like any other occult sim.
