Chicago (IL) Tribune columnist Mary Schmich wrote a speech to graduates in her column on June 1, 1997. One piece of advice was:
 
“Live in New York City once, but leave before it makes you hard. Live in Northern California once, but leave before it makes you soft.”
 
Schmich’s piece was soon falsely credited to American author Kurt Vonnegut (1922-2007), who supposedly gave the speech before graduates of the Massachusetts Institute of Technology. Vonnegut told the New York (NY) Times in August 1997, “What she (Mary Schnich—ed.) wrote was funny, wise and charming, so I would have been proud had the words been mine.”
 
         
Wikipedia: Mary Schmich 
Mary Theresa Schmich (born November 29, 1953) is an American journalist. She has been a columnist for the Chicago Tribune since 1992, winning the Pulitzer Prize in 2012. Her columns are syndicated nationally by Tribune Content Agency. She wrote the comic strip Brenda Starr, Reporter for the last 28 of its 60 years and she wrote the 1997 column “Wear Sunscreen”, with the often quoted “Do one thing every day that scares you”, frequently misattributed to Eleanor Roosevelt.
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“Wear sunscreen”
Schmich’s June 1, 1997 column began with the injunction to wear sunscreen, and continued with discursive advice for living without regret. In her introduction to the column, she described it as the commencement address she would give if she were asked to give one. The column was circulated around the Internet, with an erroneous claim that it was a commencement address by Kurt Vonnegut, usually at the Massachusetts Institute of Technology, and the misattribution became a news item when Vonnegut was contacted by reporters to comment. He told The New York Times, “What she wrote was funny, wise and charming, so I would have been proud had the words been mine.”
 
Chicago (IL) Tribune
1 June 1997, Chicago (IL) Tribune, “Advice, like youth, probably just wasted on the young” by Mary Schmich, pg. 4C:
Ladies and gentlemen of the class of ‘97:
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Live in New York City once, but leave before it makes you hard. Live in Northern California once, but leave before it makes you soft.
 
6 August 1997, New York (NY) Times, “A Column That Was Only Written, Never Delivered,” pg. B4, col. 2:
Following is the text of a newspaper column by Mary Schmich that has been erroneously identified on the Internet as a speech by Kurt Voinnegut.
   
Los Angeles (CA) Times
It’s Witty Advice . . . but It’s Not Vonnegut’s
August 07, 1997 | ROY RIVENBURG | TIMES STAFF WRITER
Will the real Kurt Vonnegut Jr. please stand up?
 
In recent days, an Internet hullabaloo has broken out over a graduation speech that he purportedly delivered at the Massachusetts Institute of Technology.
 
It began with the immortal words, “Wear sunscreen,” and wandered through a delightful, amusing string of proverbs, including:
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* “Live in New York City once, but leave before it makes you hard. Live in Northern California once, but leave before it makes you soft.”
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In reality, novelist Vonnegut had nothing to do with the alleged commencement address. And neither did MIT. The words were hijacked from a June 1 newspaper column by Chicago Tribune scribe Mary Schmich.
 
Google Groups: ba.motss
Advice for a potential new resident (Yeah, that ? again) 
Joseph Canale
2/5/99
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I like what Kurt Vonnegut said in a speech to some graduating college class:
     
Live in New York —but not so long that you become hard.
Live in Northern California —but not so long that you become soft.
   
Twitter   
swiss army wife
@theangeIemoji
“live in New York but leave before you become hard. live in north California but leave before you become soft.”
5:45 PM - 5 Aug 2013
 
Twitter
Dan Davies
@dsquareddigest
Replying to @jkbloodtreasure
@jkbloodtreasure “live in New York, but not so long you become hard; live in North California, but not so long you become soft” Emma Goldman
8:06 PM - 6 Oct 2015
 
Twitter 
valpal
@val_pallll
Go to New York to become hard
Go to California to become soft
10:49 PM - 21 Dec 2015
 
Google Books
Ask a Native New Yorker:
Hard-Earned Advice on Surviving and Thriving in the Big City
By Jake Dobkin (co-founder of Gothamist)
New York, NY: Abrams
2019
Pg. ?:
(Also printed on Gothamist, cited below.—ed.)
Gothamist
Ask A Native New Yorker: Should I Move To Los Angeles?
BY GOTHAMIST IN ARTS & ENTERTAINMENT ON MAR 15, 2019 12:05 PM
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Likewise, the city (Los Angeles—ed.) is periodically swept by cultish fitness crazes— CrossFit being the most recent—and, at certain outdoor public staircases, you will witness people actually lining up to trot up and down, over and over. All this work, and the average person in Los Angeles is probably still less fit than the average New Yorker who gets all the exercise he needs just by walking around. There’s an old saying about life in the two cities: That you should live in New York, but leave before you become hard, and live in California, but leave before you become soft. Speaking physically, this is bullshit: Humans are much better off being in shape than turning to jelly.