"A youth applicant wanting to be awarded preference points must be at least eleven (11) years old at the time of submitting an application for a big game limited quota license in the initial drawing and must be at least twelve (12) years old when in the field hunting.
A youth applicant wanting to purchase preference points only must be at least eleven (11) years old at the time of submitting a preference point only application and must be at least twelve (12) years old by December 31 of that year."
Basically you can buy points for him during the calendar year that he will turn 12. If he doesn't turn 12 until after December 31st, you need to wait until next year.