Voting Age

Some ten-year-olds are mature and knowledgeable enough to vote. Then again, some aren't. But the characteristics you list obviously aren't determiners of one's rights. A completely stupid and uneducated 18-year-old is still allowed to vote, but a politically knowledgable and opinionated 16-year-old cannot.

Some people still think Obama was born in Kenya, or is a Muslim. They still have the right to vote. Many people voted for Bush because he was the same intelligence level as them. Some people thing the Holocaust never happened, or the moon landing was faked, or the government planned 9-11. They are still allowed to vote. If anything, voting rights should be decided on a case-by-case basis. However, this would clearly be unfeasable, as whoever controlled the test would control the world.

It used to be thought that women were too stupid to vote, or would just vote with their husbands. Now, it is true that young people would tend to vote with their parents. So do old people. The number one fector in determining one's political affiliation is one's upbringing. However, the secret ballot ensures that nobody will be intimidated by anyone else to change their vote.
Different age groups generally have different needs. These needs will not be met if they have to wait to vote, as they will then have different needs. When you get right down to it, voting is about expressing your opinion, and your opinion cannot be wrong.

Children are the future. The future is now.