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?