While in theory I'd be in favor of some sort of pre-requisite to vote. Not so much in limitation as to who could vote, but in some sort of system where a relatively short period of time would be spent in a civil service position of some sort, working with the government, at a reasonable wage (although I think any appropriate volunteer time should count as well.) Require the government to have these positions open. Whether it be assisting the legislative process in the capital, or shoveling dirt in road projects, or helping out in a school somewhere or even military service. If you want to go through this process to be able to vote, they can find a job for you. Also provides a guaranteed alternative for anyone looking for some employment experience.
That's all in theory though. In practice such a system would be prone to failures on multiple levels and prone to corruption and cutting out any "undesirables" people feel shouldn't have a voice.
With that in mind I find it impossible to have any sort of limitation on who can vote. Age is understandable, as at least in this society we do have a clear cut legal line between child and adult. Those who are taken care of and those who can take care. I'd argue children shouldn't necessarily be denied the vote, but if we have to have ANY limitation, a minimum age is the least intrusive. I'd also argue that criminals, shouldn't be denied the right to a vote either. Any criminal. In a healthy society criminals will be such a small portion of the population, even in the US with its huge prison system, that any desire for them to push radical changes to the law to allow for harmful acts to be legal would be vastly counterbalanced by the rest of society voting against it. Any elective choice for them to be a notable swing vote is already going to have widespread acceptance among non-criminal sections as well. And finally any society where criminals make up such a staggeringly large portion of the population that they would be able to enact significant change as a voting bloc to themselves is an unhealthy society to begin with with something so common being punished so harshly that it likely needs to change anyway.