Didn't want to complicate things much, but it seems to me the age at which politics gets thrown into the mix with all of those other things that kids have to deal with growing up depends on what the intent of the parent is.
For example, there are those parents that feel it's imperative that their kids follow in their path as far as what political ideology their kids should adhere to. As well, there are those parents who feel their kids should decide all on their own.
It seems to me though that the practical thing to do is to expose children to politics that is up to their level of comprehension, WITHOUT personal prejudices that could influence their views. Having children get curious about politics and then have them discover for themselves (with a little objective help from mom and dad) what the whole process is about historically and contemporarily seems to me to be a fair and impartial way to go.