Im new here, maybe Im not qualified to answer either. There are a lot of different situations where you could have to defend against a headbutt, Id think largely youd have to deal with the attacker holding your arms or something along that lines.

What I would do is use any of the hold breaking ...

Have you ever seen Tombstone? When Wyatt just tears appart Johnny (tire?, played by billybob thornton).

Despite the slapping, it was a great example of bouncer style VSD.

If I'm 16 and am attacked by someone who is 18 (in the state of New York), am I subject to any legal problems? Or would it just be the person over 18, or would they at all?

I believe 16 is a minor in New York state.