If your under 18 do you get all you paid in taxes back?

It depends on your income. If your income was less than $6,300 for the year, then you will get all of the federal withholding back and most of the state. If your income was over $6,300, then you won't. 

Age has absolutely nothing to do with how much you get back. The tax rules are the same for any age from birth to death. And student status makes no difference at all.