You must be at least 16 years-old to operate an ATV with more than 90 cubic centimeter displacements. You must be at least 12 years-old to operate an ATV with an engine size exceeding 70 cubic centimeters. If you're under 16, you must be under direct parental supervision to operate an ATV. Riders under age 16 must wear a helmet at all times.
Check out these Kenvir, KY ATV/OHV resources: