“I am pushing sixty—that is enough exercise for me”

“I am pushing fifty/sixty—that is enough exercise for me” is an old joke. Mark Twain (the pen name of Samuel Langhorne Clemons, 1835-1910) is frequently credited, but Twain never said it.
 
The joke was popular in 1955 and had several versions for different ages (such as “fifty” and “sixty”). It’s not known who first said the joke.
 
 
21 March 1955, Cleveland (OH) Plain Dealer, “The Breakfast Commentator” by Wes Lawrence, pg. 19, col. 3:
Up-Hill Work
Exercise, though good for you,
At middle age gets rough.
When you are pushing sixty-two
That’s exercise enough.
AINTREE GOTT FUNN
 
24 June 1955, Edwardsville (IL) Intelligencer, pg. 7, col. 5:
When a man is pushing sixty that’s exercise enough.
 
28 August 1955, The Plain Dealer (Cleveland, OH), “The CerfBoard: A Good Racquet,”  This Week Magazine, pg. 4, col. 3:
He put his racket back in the press and has never played since. “I’m pushing fifty-five,” he (Billy Rose, 1899-1966—ed.) decided, “and that’s all the exercise I need.”—BENNETT CERF.
   
Google News Archive
30 April 1956, Newburgh (NY) News, pg. A-8, col. 2 classified ad:
When a man Is pushing sixty that’s exercise enough.
(BARD Chevrolet—ed.)
 
Google Books
Everyone’s Mark Twain
Edited by Caroline Thomas Harnsberger
Cranbury: A. S. Barnes; London: Yoseloff
1972
Pg. 161:
I am pushing sixty— that is enough exercise for me.
Source Undetermined
 
5 January 1986, Sunday Oregonian (Portland, OR), Parade magazine, pg. 16, col. 1:
Second Best T-Shirt
If You’re Pushing Fifty, That’s Exercise Enough
 
Google Books
Everything to Gain:
Making the Most of the Rest of Your Life

By Jimmy Carter and Rosalynn Carter
Thorndike, ME: Thorndike Press
1987
Pg. 314:
When you’re pushing sixty, that’s exercise enough. — Jimmy Townsend
 
Google Books
Henny Youngman’s Bar Bets, Bar Jokes, Bar Tricks
By Henny Youngman
New York, NY: Wings Books
1994
Pg. 78:
I’ve given up exercising — pushing fifty is enough exercise for me.
 
Google Books
Tweet This Book:
The 1,400 Greatest Quotes of All Time in 140 Characters Or Less

Edited by Sayre Van Young and Marin Van Young
Berkeley, CA: Ulysses Press
2011
Pg 78:
I am pushing sixty. That is enough exercise for me.
Mark Twain