The Percent of Voting Age Population is a way to calculate the voter turnout in elections. It is one of two statistics used to calculate turnout, the other being Percent of Registered Voters.
In the United States, the process of voter registration varies from state to state, and in some states it can be more difficult to register than in others. In a state with a more difficult registration process, where only those who are truly interested are registered to vote, the turnout of registered voters will be artificially high. For that reason, the percentage of the voting age population can be used to compensate for these differences. Of course, like any other statistic, it can be misleading. Since it is based on comparing voter turnout to the total voting age population, it can be inaccurate if censuses or estimates of the population are inaccurate. However, overall, this statistic is probably the best way to measure voter participation.