The EU was a bad idea
Now we are no longer in it, it seems obvious to anybody who is paying attention that it is a pretty good idea.
and the whispers are that they want to include 16-17 years olds on the electrol roll, so they can get us back in the EU for their cheap holidays.
That's one good reason for rejoining. There are others.
The rule should be that they cannot vote on matters that took place before they can became of voting age.
Rejoining the EU hasn't taken place yet. They would surely be allowed to vote on it, just as many people voted for or against Brexit even though they were not of voting age when the first referendum happened.