I am not sure why you wouldn't allow it if you are allowing adoption. I made mine real and she ages just like everyone else (Her age is tied to the child who's imaginary friend she was). She is currently married to one of the brothers and will be moving out as soon as he grows enough life fruit for the heir's Simbot opportunity.
Okay, fine by me. It makes sense. I will make the Imaginary Friend clear in the rules. Sorry for the flip-flopping.
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EDIT: I ended up rewriting that rule and also allowing household members to be ghosts. Given all the testing that Chronic did in the Immortal Dynasty, he convinced me that there is absolutely no lifespan differences with ghosts. Others have also confirmed this. Here's the rule now:
2. No household member can be a vampire, mummy, or SimBot. However, Imaginary Friends are allowed to become household members. You're also allowed to have ghosts as household members.