why is citizenship such an important voter requirement
Well honestly, if you use your brain for a minute... [ what right do you think you would have had in deciding, say, Poland's next president? Are you aware of the respective candidates' campaigns, platforms, etc? Unless you actually live in Poland, probably not. Me neither. So that quickly covers citizenship/residence.
Age: If you let babies vote, Tinky Winky might end up getting a substantial #
of votes. Or a flickering light bulb might become mayor.
For what it's worth, I'm not entirely sure that people over the age of, oh say, 75, should vote either. I dunno, people end up just voting what they've voted their whole lives, without the slightest consideration for the issues of the time, that are on the table. ]
There are no new answers.