As I understand it, youth had a chance to vote in the referendum in exactly the same way as the rest of us. Why should they have a second chance simply because certain of them couldn't be bothered originally. 'Opportunity knocks but once', a lesson we all have to learn, sometimes to our cost. It's called being a grown up!
As for Catalonia, there is no way Madrid could ignore what was an illegal referendum. If Catalonia is allowed to go its own way without much comment, a green light is being given to other like minded states both in Spain and elsewhere. Not sure an election will solve much except to infuriate the separatists who will see it as a means to snatch away all that they believe they've gained.