Source: This quote is often attributed to Michael Pritchard, but the source for variant of this quote found is much before the birth of Pritchard, so its attribution is questionable.

Variant: We don’t stop playing because we grow old; we grow old because we stop playing.
Source: Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education (1904) by G. Stanley Hall.