I think there is a feeling that it won't matter anyway as nothing will change.
I know it's a common enough feeling, but I want to shake young people when they say this (and I'm not that not-young myself). The elderly and retired are disproportionately catered for in politics, because they so reliably show up to vote. If young people showed up, even to spoil their ballots, politicians would be falling all over themselves to offer policies that catered to them. If younger people generally had turned out for the EU referendum, it would have gone the other way. How can anyone possibly believe that their vote doesn't matter when we're staring down Brexit?
Anyway, rant over. I know you weren't agreeing with that opinion, but God it bugs me in its self-defeating stupidity.