Why give 18 year olds the right to vote?

At the time the U.S. Constitution was amended to reduce the minimum voting age from 21 to 18, the United States was in the midst of a very unpopular war in Vietnam. 18-year-olds were being drafted and sent to war whether they liked it or not. The irony of being old enough to die in war you don't believe in but not being old enough to vote was brought to everyone's attention.