That's interesting. If it's "certainly immoral" for a 40 year old to sleep with an 18 year old, at what age (of the woman) does it stop being immoral? 25? 30? 35?
And what about the other way around? 18 year old man and 40 year old woman? Exactly the same or entirely different?
I would say the older you get, the more acceptable a wider gap becomes. No particular rule of thumb though.
And yes we're now in a world of equality so just as immoral for a 40 year old woman to be a sugar mama to an 18 year old. That's certainly not unheard of these days either.