No, because she is too young to make a decision that important.
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?
Yes you can.
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.
A girl should marry between the age 20 to 25 years. A girl should marry one year after regularly meeting the boy whom she loves.
In most countries, no.
Of-course they can!
No.
if they both have parents consent
With parental consent, yes.
No, both are adults and do as they wish.
Not if the live in Norway.