According to ACF rules, You are eligible to be part of a junior team (as a Junior or Junior Culinarian member) if you meet the following criteria::

JUNIOR
A junior member is a culinarian with less than three years full-time employment in the culinary profession, a student enrolled in an accredited postsecondary culinary education program, or a registered apprentice as defined by the ACF apprenticeship training program. Junior memberships is subject to a limit of four years.

JUNIOR CULINARIAN
A junior culinarian is a high-school student between 16 and 18 years of age who is enrolled in a vocational program.

In regards to qualifying as a Junior member, if the student graduates after July of 2007 and has less that 3 years of full-time industry experience, he or she could still qualify to be on the team (even though , they might be graduated before July of 2008). This Clarification was made after speaking to Aidan Murphy, CMC, ACF national Junior Competition Director.