Responsibilities are really starting
Life is beginning to happen, and this is the perfect age for the concept of civic duty to make an appearance.
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At eighteen, the concept of adulthood is starting to be realised. Most young adults are striving towards college, employment, or a family. Eighteen is the perfect time to introduce the idea of voting. It's a great first step into actively becoming a member of society. Voting puts a certain amount of responsibility on one's shoulders. At eighteen, the idea of responsibilities has finally begun to sink in. Choices matter more. The weight and consequences of voting may actually make sense to an eighteen-year old, while they wouldn't to a younger teenager.
Although adult responsibilities should be beginning at this age, many young adults neglect these responsibilities. Look at college parties and social media. Young adults do not yet fully understand the weight of their decisions.
[P1] Eighteen year olds are beginning to understand responsibility. [P2] Voting is a responsibility.
Rejecting the premises
[Rejecting P1] Many 18-year-olds do not comprehend responsibility. [Rejecting P2] Voting is a right, not a responsibility.