Reason A: Because they are the foundation for the future
Reason B: They need to be more responsible and so giving them the right to vote at an earlier age could make them start being responsible earlier on in life rather than later on (maybe you can find a percentage of teenagers who would like to be able to vote at a younger age)
Reason C: They should be allowed to vote at an earlier age only if they can pass a test showing that they actually keep up with polictics (like elective parties, people who are running for new positions, etc.)
Reason A: If teens are allowed to get jobs at 15 or 16 and then have to pay taxes on their income, then they should be allowed to vote at a younger age. They are contributing to society so therefore they should have a say.
Reason B: Responsibility. From an early age it is important to instill responsibility into children. Knowing how the government works from an early age can help young people make a difference in their community and in future politics. Every generation is the future of America and they should have a say in what will happen in their future.
Reason C: it might force teens to be more proactive and observant of things that are happening around them. Teens tend to be self centered but by lowering the age, teens would be forced to get involved in politics and act like citizens instead of like spoiled brats. They might also be able to volunteer and help politicians see issues from their point of view. Some teens get interesting in politics or other issues from an early age because of situations they might experience at that young age.
Hopefully this helped and good luck.
