Why? Brexit will impact 16 year olds for decades. A large number of the old farts who voted for it are dead now and won't have to suffer the consequences. If you're going to adjust voting ages but don't want to lower it, it makes sense that those over 75 don't get a say in something like Brexit.
And if your thought is that 16 year olds don't have the education yet to understand the implications of something so big, I'd say they're much better informed than most people over 60 who are consuming Facebook AI slop and believing it wholesale.