“Why can’t you fly now, mother?”
“Because I am grown up, dearest. When people grow up they forget the way.”
“Why do they forget the way?”
“Because they are no longer gay and innocent and heartless. It is only the gay and innocent and heartless who can fly.”

J. M. Barrie (1860-1937) Scottish novelist and dramatist [James Matthew Barrie]

Peter and Wendy, ch. 17 “When Wendy Grew Up” [Jane to Wendy] (1911)

In his 1908 sequel play, When Wendy Grew Up, An Afterthought (some which was eventually folded into the main play (1904, published 1928) in Act 5, though not these lines), this is rendered:

JANE: Why can't you fly now, Mother?
WENDY: Because I'm grown up, sweetheart; when people grow up they forget the way.
JANE: Why do they forget the way?
WENDY: Because they are no longer young and innocent. It is only the young and innocent that can fly.