A: There are some exceptions, but in general, minors are allowed to post videos online. Exceptions include: posting a video of a sexual nature or of activity that would be unlawful for a minor to engage in, children under the age of 13 posting to a site that has a COPPA restriction, and, of course, no one should be posting videos where they do not have full copyright protection and all rights associated with copyright.
A: While your situation may depend on the circumstances that you call "hacking", in general, computer intrusion is not lawful.
However, if you leave your password on a post-it note on the monitor, then that is not hacking...
A: While I doubt that you are correct that someone who has gone to the effort of successfully hiding will appreciate your finding them and respond to your sob story, it is a fact that you can sometimes find the true operator of a privately registered web site by sending the appropriate letter to the registrar, or by filing a lawsuit and sending a subpoena.
If you are sure you know who made the post, you might be able to sue that person for a public retraction.