If you can be drafted at 18, be sent overseas to kill and die for your country, you should be able to cast a ballot. That rationale got the voting age in the United States lowered from 21 to 18 in 1971 when the 26th Amendment to the Constitution was ratified.
I missed this Washington Post article when it first came out late last year. But thanks to too-occasional BRAD BLOG and Washington Monthly contributor D.R. Tucker, I was glad to catch it over the weekend.