“grown-ups always say that things are complicated.”

As We May Think (1945)

Source: When the Snow Fell

From Here to Eternity (1951), p. 668
Context: Why was it everything was always so goddam complicated? Even the simplest things was so goddam complicated when you come to doing them.

As quoted in "Special Section: Chagrined Cowboy" in TIME magazine (8 October 1979) http://www.time.com/time/magazine/article/0,9171,916877,00.html
1970s
Context: Ever since the secret trip to China, my own relationship with Nixon had grown complicated. Until then I had been an essentially anonymous White House assistant. But now his associates were unhappy, and not without reason, that some journalists were giving me perhaps excessive credit for the more appealing aspects of our foreign policy while blaming Nixon for the unpopular moves.
These tendencies were given impetus by an interview I granted to the Italian journalist Oriana Fallaci, without doubt the single most disastrous conversation I ever had with any member of the press. I saw her briefly on Nov. 2 and 4, 1972, in my office. I did so largely out of vanity. She had interviewed leading personalities all over the world. Fame was sufficiently novel for me to be flattered by the company I would be keeping. I had not bothered to read her writings; her evisceration of other victims was thus unknown to me.

"On Three Ways of Writing for Children" (1952) — in Of Other Worlds: Essays and Stories (1967), p. 25
Context: Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.

Variant: I've screwed everything up royally. I remember you saying that growing up happens when you start having things you look back on and wish you could change.
Source: City of Ashes

Source: The Autobiography of Alice B. Toklas

Source: Ida B.

"Quo Vadimus?" http://books.google.com/books?id=vvEvAAAAMAAJ&q=%22Did+it+ever+occur+to+you+that+there's+no+limit+to+how+complicated+things+can+get+on+account+of+one+thing+always+leading+to+another%22&pg=PA34#v=onepage, The Adelphi (January 1930)