I agree that a balance does need to be struck between the voter and the state in terms of who can be eligable to vote. But at the same time, I think what everyone here has really been getting at discussing a system of checks and balances that would be capable of holding stead-fast in the face of corruption, but would still be flexible enough to allow for maturity variations on an individual basis.
In my own opinion, I think the question shouldn't be: "At what age should the people be eligable to vote". but instead "How can we ensure that the people are being informed on:
A) Their [voting] rights as a citizen.
B) Their eligibility to not only vote in an election, but to also run for office (among other things).
C) How to protect both the voter and the office from abuses of both corruption and power.
Imho, I'm not convinced that there's any one correct answer, but if I were to make a decision, then I would make it multi-tiered. For anyone blow the age of, say, 15, I would require both a written questionarre on general civics, as well as an oral exam by a psychologist. After that, I would also limit them to voting in local elections.
For someone up to the age of 20 or so, I'd require the oral exam, but also recommend the they take the questionnaire as well, just so that they have a better understanding of the process.
For those above the age of 20 whom have never voted before, I would make the requirements the same as though they were under 20, but also make it a requirement that they do so before a specific cut-off date, such as 60-days prior to the election date.
Thats just my two cents, and as I said, I don't think that there's really any one correct meathod that should be used. Most certainly, there's a bunch of more effective ways that work, but are also completely dependent on the local culture.