There is no minimum age for riding a Class I ATV in Oregon. If you are operating a Class IV ATV and you are under 16, you must meet the manufacturer's minimum age recommendation. You must also be supervised by an adult if you are operating a Class I, III, or IV ATV and are under 16. Visit page 2 of Oregon's ATV Laws and Rules brochure for ATV class definitions. All riders for any class ATV under the age of 18 must also wear a helmet.
Check out these Gilchrist, OR ATV/OHV resources:
