1. You don't have to live with the long term consequences of your actions. Who cares about things like the national debt or climate change if you're not going to be around when the s*** hits the fan?
2. Diminished mental capacity and resistance to change/consideration of other viewpoints. Sure, there are some exceptions, just as there are some intelligent 17 year olds who should be allowed to vote. But since we don't allow them to, we should be consistent and not allow 80 year olds to vote as well.
3. Reduced contribution and participation in society. Why would a 76 year old care about income taxes and how they're used if he hasn't worked in the past 10 or so years?
Discus.