The age at which one is legally an adult is eighteen. When voting on issues that affect adults, it makes sense that the freedom to have a say should come when the voter is no longer a minor. Adult issues should require an adult voice.
Many of the policies that people vote for involve teenagers. These youth should be able to have a say in real-life scenarios and propositions that actually have an effect on them, versus being immobilized in the overall decision by people who will never be affected by what they are voting for.
[P1] Voting is an adult issue. [P2] Only adults should vote.
Rejecting the premises
[Rejecting P1] Not all issues involving voting revolve around adults. [Rejecting P2] Youth should have a say in issues too.