Graham , I think one of the problems with your idea is that loyal , vocal OOTs who have supported through thick and thin could be penalised, while the Thomas Cook brigade and other hospitality attendees would probably be unaffected.
Another issue would be enforcing the policy. I can think of a number of ways of getting round the proof of residency , parents desperate to get their kids into good schools have been doing it for years. So for example as an OOT with family in and around the city I personally would be using their address for proof. How far could/would the club go to police this?
I do think you're onto something though with encouraging younger locals into the ground. I can think of a fair few night matches over the last few years, Europa League quarters/semis for example where the Kop has been buzzing and there has been a good proportion of young Scousers . I do think the club can do much more to make tickets available to local young people - build links with colleges, youth clubs, football clubs etc. Sadly its probably expecting too much of the club that this could be at the expense of the tourists rather than loyal OOTs but I do feel that it's something we should be getting together to fight over and it's a shame that this thread has at times been highly devisive.
