What should the voting age be?

[TinCow]

That's an interesting argument; if I understand you correctly it boils down to: at age 18 you come of age, therefore you should have all the rights of other able adults.
But maybe look at some other age landmarks:

At age 13 a person is considered able to commit to agreements. One may now officially troll forums and get banned for it, post copyrighted or lewd content and get called to court over it. One may now chat up FBI investigators posing as other 13 year olds, too; one may now start to build up a lifetime's worth of debt.
At age 16 a person is considered able to operate a vehicle such as a car. One can now attempt to set official speed records and drive over passers by.
At age 18 a person is considered adult. One may now elect a series of disasters to strike the USA for the coming years. One may now build up debt faster than ever before, as well as demand state funds accelerate the combined debt of all your fellow citizens. One may now become the FBI agent and pretend to be 13 years old again.
At age 21 in the USA a person is considered responsible enough to spend money to buy a drink.

I think you can make a fair case that if at age 13 or 16 you are already trusted with certain liberties which enable you to inflict a lot of harm on society you might as well be allowed to vote at that age too. Also that the restriction on alcohol is kind of weird given you are considered adult at age 18, though admittedly it's obviously a relic of the past your stuck with.

The age of majority is the age at which the law recognizes you as a full, independent member of society. This means you are no longer treated differently if you commit crimes and your parents no longer have any legal authority to regulate your actions. This last one is particularly important, as minors (those under the age of majority) are usually not held to the same standards as adults when it comes to legal responsibilities. If the voting age is lower than the age of majority, then the nation is allowing individuals to vote even though they are not legally considered mature enough to handle all decisions in their own lives. If the voting age is higher than the age of majority, then the nation is preventing an individual from voting even though they are legally responsible for all of their actions. Neither of those situations seems just.

While some rights may vest in an individual prior to age 18 (and some after age 18), in nearly all places in the US (some state laws may vary) 18 is the age of majority with respect all rights of any significance, particularly guardianship and criminal law. So, 18 seems like the proper age to me in the US. If other nations have higher or lower ages of majority, the voting age should be adjusted in those nations to match the age of majority.