how is that a sad day for democracy? Young people had plenty of opportunity to vote but didn't bother. Non-participation in the democratic process is turning down your chance to back your stance.
I think what you actually mean is "sad day for my political viewpoint".
that anything less than a considerably majority participate in the vote could be considered a bad thing. Young people don't have a unique hold on political insight over older groups.
Older people also have more life experience and maybe even some wisdom. We need a diverse mix of views for democracy to be strong. Instead of penalizing one group we should try to get people more excited about voting. Things like your vote only mattering in a handful of states and there really being only two viable parties because the system suffers from the spoiler effect are obvious obstacles that most be resolved.
Your comment, however, dismissed that low voter turnout was sad at all, because they could have voted. And here you are saying the opposite -- that you understand it can be sad.
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.
I think what you actually mean is "sad day for my political viewpoint".