Children are legally permitted to have sex or drive a car in some countries, which are more dangerous and difficult than voting.
Voter turnout among youth will improve if young people get in the habit of voting before they reach 18 and go to colleges far away from their state of residency, like it did in Germany when some states lowered their voting age for municipal elections.
Education for and about democracy would be better served if there was no voting age.
Government entitlements suppress fertility, which means the youth demographic is systematically suppressed, with no political power to offset the effect.
Governments derive their just authority from the consent of the governed. To be legitimate, those who govern and those who legislate must be elected by the people, not a special subset of the people, such as those over the age of X years.
Some youth are believed by some to not to have sufficient understanding of the realities of life to participate in voting. In response to this, most youth suffrage advocates[who?] point out these are the identical arguments used against women's suffrage in the past. Likewise, mental capacity or knowledge is often not a bar to the elective franchise.
Demeny voting is the idea that parents would cast votes on behalf of their children thereby ensuring that the interests of children were properly accounted for in the voting system. Most young people do not support themselves financially and are reliant upon parents for support, thus parental voting power should be proportionate to the number of dependents, especially where government benefits are concerned, to appropriately counterbalance the interests of the childless. Essentially, a case for "no taxation without representation."