The ability to play is one of the principal criteria of mental health.

Ashley Montagu (1905-1999) British-American anthropologist and humanist [b. Israel Ehrenberg, a/k/a Montague Francis Ashley-Montagu]

Growing Young (1981)
    (Source)

In the 2nd edition (1989), Montagu rephrased this a bit less prosaically: "Play is, of course, among the clearest of neotenous behavioral traits; it is the neoteny of the intellect, a principal criterion of mental health." Montagu uses the term "neoteny" to mean "growing young."