[ZULU]

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The draft age was 18 back to Civil War times at least. The voting age was 21 back to the time of the Constitution's beginning.

I think the idea was one had to serve in the military to earn the full rights of citizenship. This concept went back as far as the Spartans and Ancient Romans.

[BikerNYC]

I think the law was changed because the Supreme Court held that you could not draft a citizen without giving him the right to vote for those who were sending him to war. Not an unreasonable proposition, IMHO.