Some article I read -- I no longer remember which one, but it isn't important -- tried to claim that pre-20th century female couples inevitably involved an age difference and therefore followed the classical model of an older mentor and a younger "beloved". It seems to me that this is a case of finding what you're looking for, because even when a couple are very close in age--for example, fellow students at a boarding school--it may happen that one is seen as the older, more experienced figure and the other as the follower.