Every year young people are different. They may have a strong feeling of being a part of society this year, they may feel completely disenfranchised next year. Because every year there's a new set of them.

Every year old people stay the same. Because their views are already set in stone, you won't convince them of anything new. Also, they had time to form good behaviors. They had time to learn to vote even if they feel disenfranchised. They had time to learn to complain if they think something isn't right (even if they're wrong about it), and that you can make a difference and have your voice heard. They had time to learn these lessons and now, even if their opinion is wrong, they'll go out and vote, they'll put in the effort to get what they want.

Unfortunately, children haven't really learned all this. The average 18 year old is still getting bullied by his McDonald's boss to come in on their free time, and indoctrinated by their bad teachers that you shouldn't question authority, of course they don't think their opinion matters. They also don't understand enough about politics to know which way they want to go on some issues. This drives them to be indifferent towards it.

Turn out for one of the parties punching down on them? Can't imagine why they haven't done it. Total fucking mystery.