Depends what you're using it for. The parent poster seemed to be implying a wish-list for a future utopian society, in which case the right denominator really is the whole population. (Or even more accurately, the set of all human beings that will be born in the future, but this causes issues with the numerator, since unborn humans don't exist yet.) The fact that children are legally prevented from voting has no bearing when you're talking about a future legal system that doesn't even allow the concept of national sovereignty; only their opinion matters. Indeed, there's reason to believe that their opinion may vary significantly from the majority opinion of people several decades older than them.