You must be at least 12 years-old and have an OHV safety certificate or be 16 years-old and have a valid driver's license to operate an ATV in North Dakota. If you're under 16 and have neither a driver's license nor a safety certificate, you can only ride on private land owned by a parent or guardian. Riders under 18 must wear a DOT-approved helmet, children under six years-old should not ride as passengers on ATVs.
Check out these Davenport, ND ATV/OHV resources:
