C.S. Lewis’ seminal children’s book series, The Chronicles of Narnia, has been highly influential in the way young people’s fantasy literature has developed. One of the key features of Lewis’ universe is that when the Pevensie children get older, they can’t return to Narnia. In the books, Aslan is the voice of reason and imparts his wisdom and many of the rules of Narnia to the children. One of these is that Peter and Susan, the oldest Pevensie children, won’t be able to come back after the events of Prince Caspian, but it’s never explicitly stated why.

https://screenrant.com/why-adults-not-go-to-narnia-cs-lewis-books/