A special license is not necessary to operate a snowmobile in Colorado. However people between the ages of 10 and 16 must be certified or under the supervision of someone who is certified to drive a snowmobile on public lands. Although safety training is not required for those with a valid driver's license, Colorado Parks and Wildlife highly recommends attending the free seven-hour training.
Check out these Arapahoe, CO ATV/OHV resources: