Example
Many argue that since at age sixteen a person is considered old enough to pay taxes, join the army to fight for their country, or leave full-time education, then surely they are old enough to vote?
While the line has to be drawn somewhere, why is 18 currently the magic number? Well, its the age at which the UN define adulthood, and in most countries the voting age is 18. Other aspects of adulthood, such as being legally dependant from your parents, and borrowing money, also happen at 18.