I don't believe there's any age or grade requirement at the College Board level; you do have to find a school willing to proctor the exam, if you aren't enrolled in a school that already offers AP testing. Because the test day is a school day (as opposed to the SAT, which is a Saturday test), if you aren't homeschooling, you'd need to make arrangements to be absent for the test period.
She also would need a valid ID. (Our school accepts school photo ID or state-issued driver's licenses; I can't imagine they wouldn't accept a state-issued ID card.)
