The best revision for the claim as a rebuttal to the counterclaim is:
Students usually study government and civics during the last two years of high school, so 16- and 17-year-olds find voting to be a way to engage with what they learn.
This revision directly addresses the concern about low voter turnout among the 18 to 29 age group by suggesting that engaging younger students in voting while they are learning about government can foster better voting habits and greater civic involvement.