Those who didn't vote cannot be counted - they have decided that they don't care one way or the other. Likewise those who are not eligible cannot be counted - they cannot vote because their voice is being intentionally ignored as not relevant (it is fair to argue they shouldn't be excluded but that is a different issue).
>it is fair to argue they shouldn't be excluded but that is a different issue
It is, but for many of them they really should be excluded. How exactly is a 2-year-old going to make an informed vote, after all? How is an infant going to even fill out the ballot?
I would argue, however, that prisoners should be able to vote.
Just a note about prisoners voting, in case anyone has any negative opinions on that:
If you have so many prisoners in your society that it's substantially affecting the outcome of an election, then your society has a real problem, and it's not the prisoners.
