There is nothing to stop it from happening. And it is probably no illegal, depending on what state or country you are in.
no
Yes if you live in Britain.
Only if the boy has parental permission or a court order.
no due to the 18 year law, you have to have a parents consent
YES
Yes
There are no laws about dating.
no not till she is 18
Yes.
Dating isn't illegal, but having sex might be.
If she was below the age of consent, yes.
In most states, yes.