I agree that's it's completely ridiculous that you're allowed to get killed (not kill other people mind you) in the military at age 18, vote for a leader who controls a very powerful nation, and destroy your own lungs at the age of 18... yet you're not allowed to destroy your liver, too??
The issue is that when men register to vote, they must also register for the draft. Stupidly, we don't even use that anymore, plus it's incredibly sexist that only men have to sign up. But, since you can be drafted at 18, that means voluntary service starts at 18. It comes from the idea of "to have the right to vote, you should be willing to defend your country!"
Honestly, all of those things should be moved up to age 21, or 20, or what ever arbitrary age they want. (Why 21 now that I think about it? That's a silly age!).
It's never going to happen, because "Oh, you can't take away the right of democracy from young adults! They need to learn how democracy works!" And so, attached to that is, "if you want to vote, then you want to serve your country, so join the draft!". It's a vicious, dumb cycle; it's outdated and it needs to be changed.
For one thing, either get rid of the draft, make it start at an older age, and include women in it if you're going to keep it. Voting can still start at 18 if people get their panties in a bunch, but raise the age of the draft to say... 20. That's a nice round number. Secondly, lower the drinking age by one year. Now it's 20, too. Finally, cigarettes are going to be illegal soon by my thoughts, but make that age 20, too. Just because.
Now kids can "learn democracy" without worrying that they're going to be pulled into a war. I really don't like the draft...