There is, IMO, such a thing as 'too young'; we did a mock election in our first-grade class, and the two most prominent reasons I recall behind why votes were cast for particular candidates were "my friend told me to" and "Jerry Brown - like Tom and Jerry!".

That being said, middle/high schoolers are entirely capable of examining issues and coming to more thoughtful conclusions. 'Capable' doesn't mean they will, but that's true at any age.

Others have posted about the lobbying/pressure aspect, which is somewhat concerning.