You do not need a special off-roading or even driver's license to ride an ATV in Pennsylvania, but you will need to register the vehicle.
Check out these Philadelphia, PA ATV/OHV resources:
Canoeing, kayaking, and paddleboarding opportunities near you
Your local trail guide for all ages and skill levels
Local fishing holes, gear advice, and retailers
Discover local campsites, helpful hints, and where to buy gear
Education, permits, regulations and where to hunt in your area
Local archery ranges, tips for beginners, and advice on the proper gear
Get started right with local safety education, training, ranges and retailers
Off-road riding in your area, plus instruction, rentals and dealers
