Old Voters, Young Voters

In the last couple of days I've read about 16 and 17 year olds in the UK who think it's unfair that they weren't allowed to vote on "Brexit."  Thus, they complain, very old people are determining a future that's mostly going to be lived by young people.  Maybe 16 and 17 year olds aren't wise and informed enough to vote, but their complaint makes me wonder: would it have been more fair if the Brexit vote had involved a multiplier, so the vote of a 20 year old counted for 1/20 and the vote of an 80 year old counted for 1/80? That certainly sounds repugnant.  What, should we count the vote of a 20 year old with a terminal illness like they were 80?  On the other hand, there's something sensible about age-based multipliers. Not that I'm recommending this approach.  Surely it's odious! I'm just intrigued by the fact that it's not obvious why it's odious.  Of course, you wouldn't need age-based multipliers if old people just restrained themselves, deferring to the young who will live for many years with the outcome of Brexit.  You'd think they would do that, to some degree--that they would ask their children and grandchildren about their preferences, before voting.  But just in case they didn't do that, you could have age-based multipliers....and then, why not for all elections?  Yeah, it's horrible and obviously undesirable, but it does seem puzzling why that's so!