I’m a Sophomore in high school. I don’t know if that counts as the age catergory you’re referring to, but in my US History class we just had a mock election, where we voted on some local initiatives, as well as voting for US president.
It wasn’t a “That sucks, vote for this” thing (for the most part.) We had formal debates on some of the local measures, which were very informative. We had days to prepare for them, and I got to hear both sides of many arguments. (And, for the record, I really kicked ass when I debated. By the time I was finished, only 2 or 3 people in the class were opposed.) It was very enlightening. I think it should be done more often.
On the other hand, when I was in first grade, we had a mock-vote between George Bush and Bill Clinton to see who would win. I had no idea who they were, except that they were running for president, and that Bush was the current president. I voted for Bush, simply because I thought everybody else would vote for Clinton based on the fact that he was a newer president and newer=better. I didn’t have the slightest clue what the issues were for either of them. But I suppose I did learn about how the voting process works.
From my experience, I’d say mock votes are a good thing if you do them right.