why is the rest of tom's life an "ani- climax"?

The quote states that Tom is "One of those men who reach such an acute limited excellence at twenty-one that everything afterward savors of anti-climax."

This really means that the height of Tom's gifts were realized at the age of 21. Tom really had nothing to give to anybody after that. He was borne rich and he would stay rich.As he aged there was nothing of any substance to him.

Tom was quite the "BMOC" at Yale having been in Nick's social club. He has married Daisy, quite a catch, and lives a wealthy lifestyle. He asserts himself in political conversations, but the truth is that much of his life is superficial, and he is not particularly happy with all of his material conveniences. He keeps a mistress in New York; he thinks his opinions should be nearly "sacred" to those who should be impressed with him. Yet the truth is, there is nothing exciting about his life now and there will not be much.