As Jane H left a comment on the more about Narnia codes post, I thought I'd write about the Susan problem as I see it.
One of the Big Betrayals in many female eyes is the transformation of Susan. In The Lion,The Witch and the Wardrobe and Prince Caspian, many female readers (including my daughter) identify with Susan, but by the time The Last Battle rolls around, Susan has forgotten about her time in Narnia and wants to be a grown up. She prefers discussing nylons and lipstick to talking about past adventures. She is also not involved in the train crash. And the whole episode leaves a bad taste. So much so that many of the Susanites refuse to re-read the books.
Some people go through this stage where they openly mock childhood pleasures. I can remember my brother doing the same thing as a teenager. We used to play various long and complicated imaginary games but then one day he grew up and then mocked. It hurt at the time. When he was suffering from his final illness in his late twenties, my brother had the grace to say that those long ago childhood days were some of his happiest and he took great pleasure from the memories of those games.
It is the Age of Unbelieiving where it is cool to pretend that you were never interested in xyz. You actually open mock and deride the thing. You want to be a grown up and to be thought to be grown. Therefore, childhood pleasures are not what is required. How long it lasts depends on the individual. It can be argued that it is a necessary part of growing up. But it also is terribly sad. To be so desperate to be thought of as big that you have to deny the pleasures of your childhood self. With maturity comes the realisation that you can take pleasure, even if you can't revisit.
To be fair to Lewis, he never says that Susan will not reach Narnia again. Aslan simply refuses to tell her story. In The Horse and His Boy, Aslan says he only speaks of an individual's story to that individual. And what ever happens, happens out the scope of the book.
So does Susan get out of her own age of unbelieving? How does she cope with losing her two brothers and her sister? Does she have children of her own and then tell them stories about Narnia? Lewis never reveals this. Given her age at the start of the books, it is possible that Susan could still be alive and telling her grandchildren or great grandchildren the stories. There again, she could have become bitter and twisted. But I like to think the former. And after a long fulfilling life, Susan is reunited with her past. once a queen in Narnia, always queen. It is just the pathway back was somewhat longer. (Or at least this is what I tried to explain to my daughter)
There again I write romance and like happy endings.