Unlikely in most countries.
No, because even if you are in love with him; he cannot marry a minor it is illegal.
I believe the answer is YES as long as the 21 year old proves to be capable of doing so.
It's perfectly normal for a 23 year old to be intrested in a fifteen year old, but they can't date until both are eighteen (it's against the law).
You seem to be having trouble figuring out which word is the noun that needs to be the plural. Which thing do you have more of - twenty, year, or old? Twenty years is the modifier for old. You have two twenty year olds.
No; that is child molestation.
yes.
He was 15 years old when he married Marie Antoinette (14) and 20 when he became King.
No, the girl must be 18 years old.
Yes if you live in Britain.
as far as I'm aware it's illegal in Britain.
No. You are not emancipated.
No, it most certainly is not legal! It would be a crime.