But an arbitrary cutoff point is arbitrarily denying the right to those who do have those faculties–maybe the average 15 year old lacks them (although where is the evidence to support that claim?), but even if that's true, certainly some 15 year olds are more intelligent/well-informed/mature than average, and likely some of those 15 year olds have greater "faculties to rule" than many people far older than them do. If, as you claim, democracy really means "all the people rule", then an arbitrary rule that no 15 year old can vote is incompatible with democracy, hence democracy nowhere actually exists at present.