well it's a little eggandchickeny

If young people know they are not allowed to vote anyway, why care??

Yes, politics, even in self-named democracies, needs foot soldiering beyond the ballot box - so even without one vote themselves, minors can do a lot of things. But still, "I don't have a vote" IS a reason for many to be uninterested in politics - and the experiences with ethnic minorities, women and other historically excluded sectors extensively proves that.