When Daisy, Tom, Nick, Gatsby, and Jordan are at the Plaza Hotel on a hot summer's day in Chapter VII of The Great Gatsby, Nick realizes that it's his thirtieth birthday. Tom has just revealed Gatsby to be a bootlegger in front of Daisy, and she starts to fall out of love with Gatsby. Fitzgerald writes, "With every word she was drawing further and further into herself" (page 134).
It is after all of the revelations about Gatsby and the sense that Daisy's love affair with Gatsby is over that Nick realizes it is his birthday. He thinks, "I was thirty. Before me stretched the portentous, menacing road of a new decade" (page 135). The use of the image of a road is metaphorical, but it also refers to the scene that follows shortly after the group leaves the hotel, when Myrtle Wilson is struck down and killed by a car. This event marks the eventual end of Gatsby's life (as Myrtle's husband mistakes him for the killer and kills Gatsby). In a sense, it also marks the end of Nick's innocence. His turning thirty is significant because he realizes that his innocence about Gatsby and his sense of the promise of life are over.
The significance of Nick turning thirty is that he now considers that his youth has passed and he is facing a lonely and unfulfilling future. Throughout the novel, Nick has been the narrator of the story; he has been more of an observer than an actor or character. He has been inspired by the passionate dream of Gatsby and in his admiration for Gatsby's determination and persistence he sees himself in a less favourable light. He realises that he does not have a dream of his own; he does not engage deeply in emotional relationships; and he does not have close friendships. Now, as he looks to his future, he contemplates a lonely and unexciting life; everything appears to be dulling and fading, everything is becoming "thin"--his hair, his friendships, his opportunities.