From the PennDOT site:
SEAT BELT LAW
In accordance with Section 4581 of the Pennsylvania Vehicle Code, it is a law in Pennsylvania that:
โข All drivers and front seat passengers in vehicles, light trucks and motor homes must wear seat belts.
โข If you are transporting passengers age eight (8) or older but less than age 18, they must wear seat belts,
no matter where they are riding in the vehicle.
โข If you are a driver under the age of 18, the number of passengers may not exceed the number of seat belts
in the vehicle.