An old joke or an old person is truly old if it or he had been around “since the Dead Sea was only sick.” “You was born before the Dead sea was taken sick” was cited in print in 1912, when it was said about an old baseball player.
   
American rockabilly musician Ronnie Hawkins (born 1935) frequently used the line, saying in 1980 that he’d been around since “Moby Dick was a minnow, the Dead Sea was only sick, and Christ was a corporal.” “That last joke’s so old that the last time I heard it the Dead Sea was only sick” was a joke in Boys’ Life magazine in 1983.
 
The joke was popularized—but not coined—by American actor and comedian George Burns (1896-1996), who wrote in a 1985 book:
 
“Trust me. I’ve been around for a while. When I was a boy the Dead Sea was only sick.”
   
   
Wikipedia: Dead Sea
The Dead Sea (Hebrew: יָם הַמֶּלַח, Yam ha-Melah, “Sea of Salt”, also Hebrew: יָם הַמָּוֶת, Yam ha-Mavet, “The Sea of Death”, and Arabic: البحر الميت‎ About this sound Al-Bahr al-Mayyit), also called the Salt Sea, is a salt lake bordered by Jordan to the east and Israel and the West Bank to the west. Its surface and shores are 429 metres (1,407 ft) below sea level, Earth’s lowest elevation on land. The Dead Sea is 304 m (997 ft) deep, the deepest hypersaline lake in the world. With 34.2% salinity (in 2011), it is also one of the world’s saltiest bodies of water, though Lake Vanda in Antarctica (35%), Lake Assal in Djibouti (34.8%), Lagoon Garabogazköl in the Caspian Sea (up to 35%) and some hypersaline ponds and lakes of the McMurdo Dry Valleys in Antarctica (such as Don Juan Pond (44%)) have reported higher salinities. It is 9.6 times as salty as the ocean. This salinity makes for a harsh environment in which animals cannot flourish, hence its name. The Dead Sea is 50 kilometres (31 mi) long and 15 kilometres (9 mi) wide at its widest point. It lies in the Jordan Rift Valley and its main tributary is the Jordan River.
   
4 August 1912, Sunday World-Herald (Omaha, NE), “Minute-Men of Baseball—Sam Crawford” by Ripley, pg. 4-S, cols. 3-4:
Sam silently listened to such remarks as: “Why, Sam, you and ‘Pop’ Time were boys together.” “You was born before the Dead sea was taken sick.”
 
4 April 1914, Denver (CO) Post, “Grizzly Pot Shots,” pg. 6, col. 2:
All of which called “Hank” Butcher to say: “That’s about the poorest I’ve heard since the ‘Dead sea” was taken sick.”
     
Google News Archive
1 November 1980, The Gazette (Montreal, Quebec), “The Last Romp: Ronnie Hawkins at 45, Shooting for movie stardom” by Earl McRae, Today, pg. 12, col. 1:
... as he (Ronnie Hawkins—ed.) likes to put it in his Arkansas drawl, “Moby Dick was a minnow, the Dead Sea was only sick, and Christ was a corporal.”
 
Google Books
February 1983, Boys’ Life, “Think & Grin,” pg. 72, col. 2:
That last joke’s so old that the last time I heard it the Dead Sea was only sick.—Alan Frazin, Phoenix, Ariz.
 
Google Books
Nothing but Winners:
Over 6,000 one-liners, alphabetized and categorized for easy reference
By Pat Williams and Ken Hussar
Wilmington, DE: TriMark Publishing Company
1984
Pg. 424:
When he was a boy, the Dead Sea was only sick.
 
Google News Archive
25 January 1984, The Gazette (Montreal, Quebec)< "Price is right for 'Duddy'" by Thomas Schnurmacher, pg. G-1, col. 5:

Singer Ronnie Hawkins, who opens next week at Le Portage of the Bonaventure Westin Hotel, has been in show-business for many years. Says Ronnie, I’ve been around since the Dead Sea was only sick.”
 
Google Books
Dear George:
Advice & answers from America’s leading expert on everything from A to B
By George Burns
New York, NY: Putnam
1985
Pg. 56:
Trust me. I’ve been around for a while. When I was a boy the Dead Sea was only sick.
 
Google Books
Laugh Off:
The Comedy Showdown Between Real Life and the Pros
By Bob Fenster
Kansas City, MO: Andrews McMeel Publishing
2005
Pg. 208:
Comic George Burns: “When I was a boy, the Dead Sea was only sick.”
   
Google Books
The World’s First QUIP THESAURUS with literally Billions of Quips
By Caleb Spalding Atwood
Bloomington, IN: AuthorHouse LLC
2013
Pg. 63:
When X was young the Dead Sea was only sick. (Milton Berle)