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.)
