Gross well says that children are young because they play, and not vice versa; and he might have added, men grow old because they stop playing, and not conversely, for play is, at bottom, growth, and at the top of the intellectual scale it is the eternal type of research from sheer love of truth.

Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education (1904), p. 235. The reference to “Gross” is apparently to Karl Groos, Die Spiele der Thiere (The Play of Animals), 1896, p. 68, "die Thiere spielen nicht, weil sie jung sind, sondern sie haben eine Jugend, weil sie spielen müssen." (“The animals do not play because they are young, but they have a youth because they must play.”)

Variously attributed to Benjamin Franklin, Oliver Wendell Holmes (Sr or Jr), Herbert Spencer, or George Bernard Shaw, but this is apparently the original source.

We don't stop playing because we grow old; we grow old because we stop playing."