Age is not a factor rather, the qualifying factor is that the student is a junior in high school.
‘Students who plan to spend the usual four years in high school (grades 9 through 12) before entering college full time must take the qualifying test in their third year of high school (grade 11, junior year).’ (page 6)
http://www.nationalmerit.org/s/1758/images/gid2/editor_documents/student_guide.pdf