The Kiddie Tax can apply to non-dependents.

If the 21 year old is a full time student, the Kiddie Tax applies unless he pays for over 1/2 of his own support with EARNED income (or he files a joint return or both parents are dead).

So if the student is disqualified from being a dependent because he doesn't live with his parents, or because he provided over 1/2 of his own support but not with EARNED income, he would still be subject to the Kiddie Tax.