Well, I think Aslan mentions that the older kids were supposed to grow closer to their own world, using the wisdom they had learned in Narnia in their everyday lives. I don't think they had outgrown Narnia exactly; they were simply better equipped to understand their own world.
Also, the older kids do make it back eventually- after death, they come to the New Narnia, in Aslan's country. The learning period was over; they had practiced in their world what they had learned in Narnia, and now, as Aslan put it, "The holidays had begun."

Posted by: @coracle

- Why don´t Susan and Peter come back to Narnia when they become old? Why can´t an adult or teenager come back to Narnia?

Like @the-mad-poet said Susan and Peter have grown old enough to see Narnia in their daily life. They were learning to be more responsible as  they were growing out of the innocence of childhood and into the reality of Adulthood. They took all they learned in Narnia and applied it to their daily life. As they grew up, Narnia shaped them into becoming better adults even without losing their faith in Aslan or Narnia itself. Narnia isn't just a place for childhood memories or games. It isn't a fantasy land or a playground for kids, it is a place for learning and growth. It is a place for trials and consequences. When you have learned everything you could, became the adult you were meant to be, well then, your time is up and you take that with you when you leave. Even though you can't come back to that place that gave you joy, it also gave you memories and people to share them with.  Aslan didn't just bring the Pevensies to Narnia just so that he could send them away again, he brought them so that they could change for the better and especially so that Narnia could change for the better. He brought them to show them that they had a purpose. Narnia helped them to grow into adults while still being a child.