I was reacting differently to your sentiment (that low turnout is generally bad) and to the OP's (that the brexit vote was a "sad day for democracy" because their side lost and younger voters didn't bother to turn out). I think that broadly, low voter turnout is bad for representation, but that if people can't be bothered to represent their side then it's not a collapse of democracy when the other side does in fact turn up to vote.

In other words, bemoaning the result of a democratic referendum as a sad day for democracy, and attributing it to your side not showing up on the day, makes no sense and plays into a long-running theme that when the left wins it's a success for democracy and when the right wins it's a failure of democracy.