There is no ATV-specific licensing requirement in Iowa. However, riders who are 12 to 17 years-old must have completed an approved ATV safety course and carry the completion certificate with them anytime they are riding. This regulation is enforced for both resident and non-resident riders. Riders under 12, or riders under 18 who do not have a safety certificate, must be under the direct supervision of an adult.
Check out these Sergeant Bluff, IA ATV/OHV resources: