If you compare Susan's treatment with Lyra's growing up, the latter has a much more positive view. Some say what happens to Lyra is negative and that Pullman is giving out the message that fantasy and fantasising are childish things (this may indeed have been davem - if not he can punish me with the washing up later...:P). But no. Without giving too much away, Lyra is told that she must stop telling lies (hence the name?) and instead tell the tales which she finds through living her life - a lesson that life is for living, that at the end of it you should have stories to tell. She is not stopped from adventuring (as shown in Lyra's Oxford and the promise of the Book of Dust), not stopped from operating in a fantastic world, but she is also not prevented from growing up. That's a fantastic message, especially for someone like me who is a grown up who loves magic and mystery. Much better than Peter Pan or Narnia where only the infantile are allowed access to magic.
Likewise with what JK Rowling does with her young female characters. They are allowed to have boyfriends and learn about being grown up while still operating in a magical world, and what's more she makes it cool to be clever.