I think this 40-year cycle for nostalgia is too arbitrary, but it starts from a good point (via The New Yorker):
That takes us to the current day, and, at last, to the reasons behind the rule. What drives the cycle isn’t, in the first instance, the people watching and listening; it’s the producers who help create and nurture the preferred past and then push their work on the audience. Though pop culture is most often performed by the young, the directors and programmers and gatekeepers—the suits who control and create its conditions, who make the calls and choose the players—are, and always have been, largely forty-somethings, and the four-decade interval brings us to a period just before the forty-something was born. Forty years past is the potently fascinating time just as we arrived, when our parents were youthful and in love, the Edenic period preceding the fallen state recorded in our actual memories. Although the stars of “Meet Me in St. Louis” were young, and its audience old and young both, Vincente Minnelli, its director, was born in 1903, just a year before the World’s Fair he made into a paradise. Matthew Weiner, born in 1965, is the baby in his own series.
And this is a good closer
And so, if we can hang on, it will be in the twenty-fifties that the manners and meanings of the Obama era will be truly revealed: only then will we know our own essence. A small, attentive child, in a stroller on some Brooklyn playground or Minneapolis street, is already recording the stray images and sounds of this era: Michelle’s upper arms, the baritone crooning sound of NPR, people sipping lattes (which a later decade will know as poison) at 10 A.M.—manners as strange and beautiful as smoking in restaurants and drinking Scotch at 3 P.M. seem to us.