I know everybody is different, but I'm just giving my personal example, that at 16 I didn't know a thing about politics, was not overly interested in the subject, and would either not have voted, or perhaps just done what my parents did.
18 is the age at which you become an adult and have to start making (if you haven't already) adult decisions - having a say in how the country is run is one of the most adult things I can think of in our society as a whole. Children and teenagers are, as a general rule, more self obsessed than adults. This is not necessarily a bad thing, but it does mean, in my opinion, that most of them wouldn't (and shouldn't) be thinking of these issues. Haven't they got enough on their plate at 16? Let them be teenagers and worry about teenage things - there's still two whole years for them to discover more about politics and form an opinion about where they stand.