I'm puzzled by your take on that saying. I've always thought it meant (and mb I'm wrong but I think I'm in good company) if the mother looks good then it's likely the daughter will at her age too, thus the physical attraction will still be there in later years.  And inversely, if the mother is unattractive, then buyer beware, it's likely the attraction will fade. Maybe I'm reading your post wrong but you seem to feel it implies the mother will be unattractive just because she's older. A woman/man can be older and still be very attractive, in fact some people look better as they get older.