The warning to young people to "not trust anyone over thirty" was not an original realization of the American counter-culture movement of the 1960s. Since at least the second decade of the twentieth century, age thirty separated youth from full adulthood, freedom from responsibility, and adventure from mundane routine.
The 1920s were perhaps the first decade to feel the full loss of waning youth. Prior to the turn of the century, most young people of modest means were either working in agrarian jobs or employed in factories. Rural young people tended to marry quite young. Women, prior to the twenties, rarely worked outside the home. World War I, which ended in 1918, had a sobering impact on both men and women. The fervor of the Second Industrial Revolution, (sometimes called the "Technological Revolution") was winding down but it had brought many men and women from the farms to the city. Young people now had some extra money to spend on themselves. The proximity of the sexes and extra cash were essential in creating a vibrant nightlife, which also led to a drastic change in feminine mores. The "flapper" was born in the twenties. Women cut their hair (traditionally a symbol of maturity) and wore short skirts (in defiance of modesty). Men felt the social change as well. Many delayed marriage and instead spent their extra money on drinking and dancing.
But one can only avoid responsibility, seemingly, for so long. There is always a new crop of young people nipping at the heels of the older generation, urging them off the stage. Nick is beginning to feel this pressure: "I was thirty. Before me stretched the portentous, menacing road of a new decade," he tells himself.
F. Scott Fitzgerald, like his character, was feeling the pressure of aging. In 1925, the year of Gatsby's publication, the author turned 29. It is interesting to see how he amended the line cited above. Susan Bell, in her essay "Revisioning the Great Gatsby" (which can be found in her book The Artful Edit (2008)) traces the evolution of Nick's declaration. First, Fitzgerald tries out this sentence:
I was thirty. Beside that realization their importunities were dim and far away. Before me stretched the portentous menacing road of a new decade.
Later, he amends it to read:
I was thirty—a decade of loneliness opened up suddenly before me and what had hovered between us was said at last in the pressure of a hand.
But finally, Fitzgerald settles on this:
I was thirty. Before me stretched the portentous, menacing road of a new decade."
Later, in the taxi with Jordan, Nick explains that "portentous, menacing road":
Thirty—the promise of a decade of loneliness, the thinning list of single men to know, a thinning briefcase of enthusiasm, thinning hair. But there was Jordan beside me, who, unlike Daisy, was too wise ever to carry well-forgotten dreams from age to age. As we passed over the dark bridge her wan face fell lazily against my coat’s shoulder and the formidable stroke of thirty died away with the reassuring pressure of her hand.
Thirty was a "formidable stroke" but it hasn't knocked him, or his group, completely off the stage yet. It's coming, they all know it is, but like a bar that decides to stay open an extra hour, Nick will order another drink, and stay until he is kicked out.
Nick sees his 30th birthday as a significant entrance into a world of “loneliness, a thinning list of single men to know, a thinning briefcase of enthusiasm, thinning hair.”
I would add to that the fact of it going into the depression.. the fact that Fitzgerald didnt know when he was writting this book, but he knew that the 'fun' wasn't going to last forever.. its basically forshadowing the 30's or the depression :D