Everyone is eventually too old to go to Narnia, but they stay friends. The idea is that they will get to know Aslan (Jesus) in their own world.
Susan is not damned. She’s still alive; her story isn’t over. She has decided to leave Aslan and Narnia–she denies it ever happened and calls it silly pretend games. She has become interested in superficial, materialistic things–denoted in the book as lipsticks and invitations–but it’s not that she uses makeup and therefore is a bad person, it’s that she’s decided to pursue shallow things over the reality of Aslan.
Now, since she’s only quite a young woman, she has plenty of time to grow up and change her mind. C. S. Lewis was thinking a lot of himself as a young man when he wrote this about her; he considered himself to have been pretty shallow and superficial at one point in his youth too. So she’s not damned–she can’t be, she’s not done yet. She has every possibility to choose her path.
Edmund betrayed everyone, yes. He left too. And then he repented and came back. Each child lives an example of a certain kind of faith or problem–Edmund is the repentant sinner, Peter is the High King–sort of like an apostle, Lucy is childlike, joyous faith (“except ye be converted, and become as little children…”*), and somebody had to leave for good (as of the end of the series), so that’s Susan.
Once you read them, you might enjoy checking out Planet Narnia, which analyzes what the author thinks is the plan Lewis used for the books. It helps if you really like Lewis and have read his space trilogy, though; it’s on the academic side. I loved the book and think he’s correct in his analysis.
*Matt. 18:3