Many institutions define adulthood as more than merely passing a milestone of age and physical maturity. Voting is one of them. But why exactly? What aspect of the responsibility to decide our own political leadership is thought to be beyond the capabilities of, say, a fourteen year-old but not an eighteen year-old? Certainly adolescents (and younger children) are prone to rash decision-making and overt manipulation by adults. However, even people decades into adulthood do not seem immune to demagoguery and emotional reasoning over rationality.
I honestly think that an age limit for voting only exists as a kind of earned privilege, a reward for growing up as it were, rather than as a necessary requirement for systematic coherence in self-government. If the most important part of voting is representing the voice of every citizen, then we ought to consider allowing anyone who understands the concept of voting to exercise that right. At the very least it’s worth a debate.