US citizens are required to pay taxes on global income, regardless of where they live. The US is unique in this regard. Why would US citizens not continue to have the right to vote while out of country? Certainly, if they renounce their US citizenship (and hence, the ability to be taxed as a non citizen non resident), they lose their right to vote.
So you're in favor of exempting minors from federal taxation?
After all, their income is basically a rounding error economically and most don't make enough to pay net federal taxes so it might even be a net loss. There's no real reason to tax them unless it's some perverse Cartmanic exercise in making them accustomed to it.
"No taxation without representation."