I think that a rule everyone should follow is if you could of had them (started pruberty think 10 years) its a no go
As long as she's legally an adult it's not too much of a gap. And I mean what I say when I say "legally an adult". Just because a person is at the age of consent doesn't mean it's right.
Not necessarily but it's case by case. Some 18 yr olds are very adult, others are still children basically. I think if your over 21 you should stick to people over 21 but that's just me. It clears out any potential problems with age and experience gap.
Of course not I have done the same myself