Just a quick Google:
Population
Persons under 5 years, percent, April 1, 2010 6.5%
Persons under 18 years, percent, July 1, 2016, (V2016) 22.8%
Persons under 18 years, percent, April 1, 2010 24.0%
Persons 65 years and over, percent, July 1, 2016, (V2016) 15.2%
59 more rows
U.S. Census Bureau QuickFacts: UNITED STATES
https://www.census.gov/quickfacts/fact/table/US/PST045217
It seems that the population over 18, as that is the age for rifle ownership, is about 70%