When at age 16, you have two years until you reach the age of majority and are fully subject to all the glories of the government in charge. Taxation is a big one that comes to mind, as well as conscription.
To me, that's akin to saying legal immigrants have a right to vote before they are granted citizenship and subject to all the responsibilities of citizenship. If you don't like conscription at 18, you can vote to change it when you reach 18 and keep those younger than you from being conscripted. Just like I have to suck it up with any number of things that I can vote to change. I have to deal with them now, but if I and enough others vote to change them, others down the line won't ever have to be subject to them.
Otherwise we might as well change the voting age to 12 and also allow anyone with feet dry in the country to vote because at some point down the line they may have skin in the game.
Also, to respond to Chris's post above: While I fully believe Chris works with some responsible and mature 16 year olds, I guarantee that for every one of them, there are nine other 16 year olds who will vote whichever way MTV tells them to, or whichever way gives them the surest chance of getting laid, or whichever way will ensure "free stuff", because they have no experience in having to fend for themselves. Most 16 year olds are immature knuckleheads. I know - I was one of them.