18.
but I also favor offering a complicated civics exam. and anyone who can pass it, whether they're 4 or 17, should be eligible to vote.
That's a great idea. Otherwise, the answer should be 17. That is what it was in Ancient Rome and that's when I first became truly interested in politics. Actually, I first became interested at 13. The issues that got me interested in politics was wanting more legal and finacial protection in theraputic cloning and protecting the environment and increasing industrial productivity with nuclear power. At that time, I felt that I was either a conservative democrat or independent or a moderate republican. It was the PATRIOT ACT that pushed me to the center-left and Abu Ghraib that made me a Liberal Democrat.
Actually, 17 is a good age to start voting because your future is finally being affected by the law. By that, I mean that kids planning for college should have a say in how much they will have to pay the banks for letting them barrow money for college.