My fifth-eighth generations had the IF gene, after the fourth generation married her mum's imaginary friend's son (complicated!). The fifth generation also married her IF. It's in the HOF, so it can be done with IFs.
I don't think I had any aging problems with it, but like Seabody said they've happened in other dynasties. I saved prior to trying for baby every time there was an IF or IF descendant involved, just in case I had a glitch like that. I think a couple of other dynasties have done it, too.