“Old soldiers never die—they just fade away” is an old saying that was popularized by General Douglas MacArthur (1880-1964) in his farewell address to Congress on April 19, 1951. Many parodies of the saying have been made.
 
The mathematician version has several variations. “Old mathematicians never die—they just go off on a tangent” was cited in 1989. “Old mathematicians never die, they just disintegrate” was cited in 1991. “Old mathematicians never die; they just lose some of their functions”—perhaps the most popular variation—was also cited in 1991.
   
“Old math teachers never die—they just become irrational” is a related saying.
 
         
Google Books
Jokes:
Outrageous Bits, Atrocious Puns, and Ridiculous Routines for Those Who Love Jests
By Paul Dickson
New York, NY: Delacorte Press
1984
Pg 67:
Old mathematicians never die, they are just nonplussed.
 
Google Books
Get Thee to a Punnery
By Richard Lederer
New York, NY: Dell Publishing
1989
Pg. 84:
Old mathematicians never die—they just go off on a tangent.
     
Google Groups: rec.humor
Professions’ Superlatives
Mark Israel
1/15/91
(...)
Old mathematicians never die, they just disintegrate
 
Google Groups: comp.lang.perl
Jump tables?
James Brister
4/19/91
(...)
James Brister                             .(JavaScript must be enabled to view this email address)
DEC WSL., Palo Alto, CA           {uunet,pacbell,pyramid}!decwrl!brister
“Old mathematicians never die; they just lose some of their functions.”
 
Google Books
Comic Sections:
The Book of Mathematical Jokes, Humour, Wit, and Wisdom
By Des MacHale
Dublin: Boole Press
1993
Pg. 39:
Old mathematicians never die - they merely decay exponentially, or should it be, old mathematicians never die - they merely tend to zero?
   
Google Groups: rec.humor
Canonical List of Old * Never Die, they just ...
.(JavaScript must be enabled to view this email address)
2/2/93
(...)
OLD MATHEMATICIANS never die, they just disintegrate.
 
Google Groups: rec.humor
Canonical List of Math Jokes [3/4]
Michael Cook
10/4/94
(...)
Old mathematicians never die; they just lose some of their functions.
   
Google Groups: rec.humor
Canonical List of Old * Never Die, they just ...
.(JavaScript must be enabled to view this email address)
1/2/96
(...)
OLD MATHEMATICIANS never die, they just disintegrate
OLD MATHEMATICIANS never die, they just go off on a tangent
OLD MATHEMATICIANS never die, they just lose some functions
   
Google Books
The Universal Book of Mathematics:
From Abracadabra to Zeno’s Paradoxes
By David Darling
Hoboken, NJ: John R. Wiley & Sons, Inc.
2004
Pg. 127:
Old mathematicians never die; they just lose some of their functions. —Anonymous
 
Twitter
Rschrim
‏@Rschrim
My old math teacher says: Old mathematicians never die; they just lose some of their functions. There WAS a collective groan in the room.
1:18 PM - 15 Apr 2008
 
Twitter
Pigsback
‏@Pigsback_IE
Old mathematicians never die, they just disintegrate
10:10 AM - 30 Jun 2009
 
Twitter
Brian
‏@nlnnet
Old mathematicians never die, they just go off on a tangent
9:00 AM - 15 Sep 2009
 
Twitter
Anon Anon and Anon
‏@AnonyMitch
old mathematicians never die. they just approach their limits.
2:18 PM - 12 Dec 2009
 
Google Books
The Mammoth Book of One-Liners
By Geoff Tibballs
London: Constable & Robinson Ltd
2012
Pg. ?:
Old mathematicians never die, they just go off on a tangent.