If you don't know whether or not they're women, shouldn't it be "Sont-ils"? Even one male in an arbitrarily large group makes the collection pronoun "ils" instead of "elles", doesn't it?
Someone mentioned earlier (I forget who) that it could be asking whether a group of girls are old enough to be considered women?
Sont-elles des femmes ou sont-elles des filles?
Why wouldn't it be both sont-elles AND sont-ils if we don't know if they are women?
Why do you say Sont-ce des femmes? or Sont-elles des femmes? Why is the - necessary?
I think it's always like that in questions :)
They implies an unknown, so it should be Sont-ils. Why Sont-elles? Wishful thinking?
I guess you are assuming they are women (hence you ask if they are women, not "what gender are they".