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.”

In The Great Gatsby, why is Nick's thirtieth birthday so peculiar to him?

F. Scott Fitzgerald was born in 1896, and The Great Gatsby was published in 1925. Fitzgerald would have been almost thirty when he published the novel. Perhaps he is really speaking for himself through his character, Nick Carraway. Nick's feelings about turning thirty may seem peculiar to him, but they are actually very common. There is something about beginning a new decade in life--whether it be thirty, forty, fifty, or beyond--that has a sobering effect on people. It makes them stop and wonder what they have been doing and where they are going. Turning thirty may make a person feel very old, but other people regard it as very young. Turning forty can be much worse. It is the entry into silent halls of middle age. The great expectations of youth have probably not been met. Forty seems like the start of a slippery slope leading to the really unsettling age of fifty.

Fitzgerald had a strange career. He became a successful writer at a very early age. He was making a lot of money and, like a lot of people during the Roaring Twenties, thought the good times would never end. He was not only selling novels but numerous short stories to the "slick" magazines like Collier's and The Saturday Evening Post. His short stories show that he was in danger of becoming a hack. His downfall coincided with the end of the economic boom and the beginning of the Great Depression. The public was no longer interested in reading about the high life of the rich.

Fitzgerald was both fortunate and unfortunate in achieving literary fame and fortune at such an early age. He became a heavy drinker, and this problem haunted him for the rest of his short life. He died when he was only forty-four. At that time he was working as a Hollywood screenwriter, but he was getting such a bad reputation for his drinking that his livelihood was in jeopardy. Budd Schulberg, who worked with Fitzgerald in Hollywood, writes about his rise and fall in his novel The Disenchanted. Ernest Hemingway knew Fitzgerald well and writes extensively about him in his memoir A Moveable Feast.

In The Great Gatsby, why is Nick's thirtieth birthday so peculiar to him?

What makes Nick's thirtieth birthday specifically peculiar is that he had forgotten it. He only realized that it was his birthday after the confrontation between Jay Gatsby and Tom Buchanan over Daisy had ended. He only became aware of the fact that he had reached this milestone when Tom asked if he wanted to have some whiskey after Daisy had left with Jay on Tom's instruction. He, almost painfully, declares close to the end of chapter 7:

“No . . . I just remembered that to-day’s my birthday.” 

The fact that this important event had slipped his mind also suggests that Nick had been so involved and caught up in the lives and dramas of others, that he had forgotten about the major events in his. He was distracted by the events unfolding around him and in which he had become an essential participant. During this period, he befriended and admired Jay Gatsby, became a guest in the Buchanan household, arranged a meeting between Daisy and Jay, and had grown attached to Jordan Baker. As such, he was both wittingly and unwittingly drawn into the drama which was to unfold.

Nick's perception of turning thirty provides much food for thought. He muses that:

I was thirty. Before me stretched the portentous, menacing road of a new decade.

Nick's observation leads us to believe that he viewed his aging as a dangerous new journey, possibly something that he was not quite ready for yet. He later also observes as he, Jordan and Tom are driving back to the Buchanan house in Tom's car:

Thirty — the promise of a decade of loneliness, a thinning list of single men to know, a thinning brief-case of enthusiasm, thinning hair.

The irony in what Nick thinks is obvious. The word 'promise' usually signifies something good to look forward to, but all Nick sees is misery. He is clearly fearful of an older future and believes that, metaphorically and literally, he is entering a new time frame in which he will lose much: friends, vitality and hair. He paints a sad preconception that is common amongst many people. As they grow older, they are driven to despair, for they believe that old age is a burden. In this regard, we can sympathize with Nick for he is no different to them.

Nick's perception provides a poignant look into how the fear of aging stifles so many and that there is always a hankering to the past. The desire to repeat that which had gone before becomes, as it was with Gatsby, a search for the holy grail - a foolish fantasy, as Nick told him in chapter six:

“I wouldn’t ask too much of her,” I ventured. “You can’t repeat the past.”

“Can’t repeat the past?” he cried incredulously. “Why of course you can!”

He looked around him wildly, as if the past were lurking here in the shadow of his house, just out of reach of his hand.

Perhaps the last line in the novel best sums up Nick's sentiment on the issue:

So we beat on, boats against the current, borne back ceaselessly into the past. 

What was significant about Nick's 30th birthday in The Great Gatsby?

Those of us that turned 30 can attest that it is a point of no return. There is a time to put away childish things and Nick realizes that he has been passively watching these people act recklessly and carelessly. They can afford to keep on doing so and probably will. He is not in that class however. Since, as he asserts, he is one of the most honest people he knows, we can assume that he will step away from this mess soon. He will leave the corrupted Jordan Bakers of that world behind.

The timing of his realization is poignant too. The plot has moved past a point of no return as well. Words have been spoken that cannot be taken back. Actions have to be accounted for. It is a “grow up” moment for them all.

----besides what is more depressing than seeing a 35 year old acting like he is 25?

-Holden Caulfield.