A 2011 YouthStyles survey gives a breakdown of the proportion of 12-17 year olds in the USA who had ridden an ATV at least once during the last 12 months and their patterns of helmet use. Of the 831 youth respondents, an estimated 25% reported riding an ATV at least once during the past year. Unfortunately, only 45% of riders say they always wore a helmet, and 25% reported never wearing one.
The most frequent riders had the lowest consistent helmet use. 8 of 10 youth who rode at least 6 times during the past year report not always wearing a helmet.
Source: Injury Prevention, February 2015