Is 60 really the new 40? It all depends on how you define the middle years of life, says author PATRICIA COHEN. According to Cohen, traditionally, society has viewed the age of 40 as the entry point into mid-life, but that perception depends on who you are. For instance, men think middle age starts earlier than women do and, not surprisingly, the older people are, the later they say middle age begins. In her new book, “In Our Prime: The Invention of Middle Age,” Cohen shows how technical advances, social change and shifting tastes in beauty and fashion have redefined what we consider middle age. And the good news is that more of us see it as the happiest time of our life rather than a downhill decline in health and fulfillment.