In Pennsylvania, you don't need a motorcycle license to ride a scooter if its engine is under 50cc, but you must have a valid driver's license. If you are older than 25 and have had a motorcycle license for two years or more, you don't need to wear a helmet.
In New Jersey, you need a motorcycle license to ride a scooter. Helmets are required for all riders.
SOURCE: Sean Theobald of Philadelphia Scooters in South Philadelphia.