With parents permission but seriously too young
No that is illegal.
An 18 year old girl can marry an 18 year old boy even if they are not in love in this state.
In most countries, no.
Of-course they can!
legally no that is basically robbing the cradle, if you know what that means, and why would you want to marry a 18 year old girl if your a 15 year old boy anyway?
No.
if they both have parents consent
With parental consent, yes.
Not if the live in Norway.
No, both are adults and do as they wish.
Yes, if the minor has parental consent.
Possibly it depends on where you live and if the parents allow it.