One of the requirements in order to have a postpaid plan is to be at least 18 years of age (regardless if you are in school or not).
I always advise people not to get a plan because of the phone but get a plan because you need it as the phone is not really subsidized. If you really want the phone, it is cheaper to get it from outside and with terms as well.
If however you really need the plan and you are qualified and don't mind paying for the phone, then aside from being at least 18, submit a valid ID and proof of income so that Globe could process your application right away.
If however you just need the plan without the phone, Globe could give you a discount (around 20%) provided you agree to be locked in for at least 24 months.