Actually, Turtle, its more than just the law, its the Constitution.
We'd have to amend the Constitution to implement any such thing, and it would be a major departure from current Constitutional law, where all you have to be is 18+ to vote.
Now I have in the past, suggested that maybe universal franchise wasn't the best idea in the world. I still wonder about that sometimes. Sometimes I indulge in speculation about other methods, where everyone could EARN the right to vote, but it wasn't automatic.
This method however, is simply no good. There are people who work their butts off, and contribute to society through their hard work, who pay no income tax. This would include a good many of our young soldiers, btw.
Even if you acknowlege that it isn't just income tax, that we'd have to look at what someone paid in SS/Medicare/FICA, property tax, sales tax, and so on, (and then we'd probably just be looking at the bottom 10% of the economic scale, for net tax nonpayers), I'm still not convinced this is a good idea.
There are other, better ways to go about doing this. I agree that the majority should not be able to raise taxes on the minority, while keeping their own negligible. There are other venues through which to pursue this aim.