@Otong777 They can easily create life on a whim. Maybe they have a way of extending lifespans or changing races? (I assume witches and humans are of different races)
@Snakekid1, 'underage' is different in different places. 16 is old enough in some, 18 is still too young in others. (Bahrain and Portugal are both 21, for example)
So the premise is that mother and daughter are easily mistaken for being the other way around.
Not exactly an original concept. But more importantly, can anything actually be done with it?