11 C.F.R. § 110.18

Voting age population.

            There is annually published by the Department of Commerce in the
            Federal Register
             an estimate of the voting age population based on an estimate of the voting age population of the United States, of each State, and of each Congressional district. The term
            voting age population
             means resident population, 18 years of age or older.

[68 FR 457, Jan. 3, 2003]

Official source