Why do we say "Life begins at forty"?

Meaning:

Maturity is the best time of life

Background:

This much used phrase (by those of us who are over 40) was the title of a book published by American author W.B. Pitkin in 1932. Unfortunately, the phrase seems to have outlived its author as although there are plentiful references to the expression, the world wide web failed to reveal any information about the man himself.

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