Does someone under 18 have to give a witness statement to the police if they dont want too?

No one can be compelled to give a statement to the police, regardless of age. You can be subpoenaed (ordered to appear in court or some other location) to give a statement under oath at a deposition or before a grand jury. If you refuse to respond to the subpoena, you can be held in contempt of court.

Added: . . . or if a material witness to a serious offense and refusing to cooperate, risk being charged with 'Obstruction of Justice.'