That's the problem: if the spare one is traveling to the future alone, she ages there at normal speed, meanwhile all the other relatives, left at home, do not age at all. So the spare can go to the future as, say, young adult, spend all her life in the future till her late elder age -- and then return to the homeworld, where no one else have aged a day. Technically she would be "(time)traveling", not moved out; the game would still consider her as a part of the household all the time. But it seems like a too convenient way to age redundant Sims much faster than anyone else in the family.
This is not an issue with abroad traveling from WA, as Sims do not age while vacationing. But time travel is different in this aspect. I hope Challenge team will have a look on that as well, to avoid the confusion.
Anyway,
Pam, thank you for the reply!
Sherry, aging should not be prolonged or shortened in any way, with the only exception of Ambrosia for immortals. As far as I can tell, it's one of the defining rules of the challenge.