Remember that much of our current system has been a slow increase of the "popular" vote, from landholders to men to adults. At some point it'll go to "everyone above the age of 0" but let's not ignore that kids will at first vote the way their parents do, and then vote the exact opposite for awhile.
I don't think we need to assume that actual children will someday vote. It's not punitive to say that we should have some age threshold into adulthood and its responsibilities. There are so many facets of society that necessitate some degree of protection of young people that it isn't unfair or unrealistic to also create a barrier around their full participation in our Democratic society.
