The brain isn't fully developed until 25
Teenagers do not have a fully developed brain, and so should not be expected to vote rationally.
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Several studies show that the brain isn't fully developed until the age of 25. The frontal cortex is the last thing to finish developing, which is essentially responsible for what we know as "teenage behavior" - actions that continue from high school, into college, and even into a person's twenties. Because of this late development, the minimum voting age should be raised to 25. It should be a requirement to have a fully developed brain before one is fully allowed to make decisions for society as a whole.
Although the brain isn't fully developed, it's important to expose young and growing minds to politics as early as possible. Even if mistakes are made, they are incorporated into the system and are able to learn from previous errors to fully solidify a political stance in the future.
[P1] The brain isn't fully developed until 25, altering decision-making skills. [P2] Accurate voting requires logical decision-making.
Rejecting the premises
[Rejecting P1] The decisions are not necessarily bad. [Rejecting P2] Decisions made before 25 still matter, and still count.