When traveling through other states and stopping for other than the listed exceptions (Food, gas,& overnight accommodations) one must be driving straight through and abiding by the laws of each state including but not limited to age, permit/no permit, CC/OC.
Right to Interstate Transportation: http://www.gunlawsbystate.com/#!/interstate-transportation/right-to-interstate-transportation/
OP...your PPP gives you NO extra "rights" or "protections" for anything other than the actual purchase of a pistol from a private individual in the State of North Carolina since you are under 21. Technically, you can purchase a pistol from an out of state private individual and have no trouble from the State of North Carolina. This doesn't mean you wouldn't from the state you purchased the pistol, however. (Proviso: IANAL)
Best advice is read, read, read...or contact an attorney that specializes in firearms law...or both.