Your case seems special.
Can you supply more info. Why is it that you haven't enlisted at
24? You should have received your Enlistment Notice 6 years
ago.
Have you had previous deferments which expired?
Did you have any problems that prevented you from enlisting earlier
on? (e.g. Physical injury, Illness, Hospitalization, Prison, Lived
Overseas)
The fact is CMPB should have called you up for NS at 18, 6 years
ago.
Did you make any effort to contact CMPB about this
earlier?