Epistocracy, no thanks. Democracy is not desirable in and of itself; there is nothing worthwhile in the idea of "one person, one vote" besides the pragmatism behind it.
One worthwhile aspect of 'one person, one vote' is that it is the political expression of the ethical viewpoint that every person counts and no person counts for any less than any other. "I'm backward, I know little and still I'm as good as you." Every person is of worth in him or herself and not merely of value in relation to ends pursued by others or by the State. It is the kingdom of ends argument.
Another argument is that the vote symbolises a social contract. "You want to govern me - first let me give you consent." So one non-consequentialist argument is that my vote is a right contracted in exchange for rights that the State has over me. Degrading my vote will erode the State's authority. Why should I obey when I have no say?
There have been 'natural experiments' of epistocracy in which those who are better informed and better educated have more votes than others. For example, boys used to get a much better education than girls in the UK and as a result men ended up with more votes than women in adulthood. It was easiest just to give the women no votes at all. Some women appeared content with this situation and unobtrusively ceded political power to those with supposedly better minds and a clearer view of world affairs. So that is one experiment that has been tried. I am not sure that the results were very encouraging for epistocracy in the long term.
Chany: "I'm sorry, but if a group of people are learning how to do basic addition and subtraction, then they don't deserve to vote on how our society is set up."
Tit for tat. If a group of people wish to deprive another group of people of a basic civil right - the vote - then
they don't deserve to vote on how our society is set up. We can all think of excuses to deprive each other of votes. The point of a democracy is that millions of people can live together in peace despite these reasons. So in a democracy I will allow epistocrats to vote (fools that they are) and you will allow the illiterate and innumerate to do so. But you are right that I don't have a good reason to exclude minors from voting, although it leads to absurdities - it's a weakness in my position.