W.H. Auden, from his “Reading” essay in The Dyer’s Hand:

Between the ages of twenty and forty we are engaged in the process of discovering who we are, which involves learning the difference between accidental limitations which it is our duty to outgrow and the necessary limitations of our nature beyond which we cannot trespass with impunity. Few of us can learn this without making mistakes … When someone between twenty and forty says, apropos of a work of art, “I know what I like,” he is really saying “I have no taste of my own but accept the taste of my cultural milieu,” because, between twenty and forty, the surest sign that a man has a genuine taste of his own is that he is uncertain of it.

Lorrie Moore, from her novel Anagrams (via Vintage & Anchor):

Basically, I realized, I was living in that awful stage of life from the age of twenty-six to thirty-seven known as stupidity. It’s when you don’t know anything, not even as much as you did when you were younger, and you don’t even have a philosophy about all the things you don’t know, the way you did when you were twenty or would again when you were thirty-eight.