Everything is O.K. as long as any authority figure has absolutely no idea it's taking place
If the 16 year old girl lives in a country where it's legal, then it's fine to date an eighteen year old boy. If she lives in America, she would have to wait for her eighteenth birthday. If he really loves her, he would be able to wait two years.
it's okey only if her father don't have any idea.
If it's ok with you, then who on earth am I to judge it? If it makes you happy do it, if it doesn't, then don't.
Yep, fine, no problem. Just dating, absolutely no sex.
If the age of consent applies, it's fine. Some might view it as morally wrong, but there's no need to listen to their opinion.