This section contains speculative information. This information has not been confirmed. Please keep this in mind.
In "The Sponge Who Could Fly," SpongeBob accidentally cause accidents for Old Man Jenkins, causing him to change his profession, from Farmer to Sailor, and then Cannonball, where he shoots SpongeBob down.
In "High Sea Diving," Old Man Jenkins tricks the Bikini Bottomites into believing he's King Neptune and proclaims himself as ruler of the sea. However, when the real King Neptune appears, he was not impressed with the lies Jenkins proclaimed. As punishment, Neptune zaps the deluded old man with his trident, burning him to a crisp.
His name is a character name used in various episodes as a running gag. His name is first mentioned in "Sailor Mouth." The name was originally applied to the elderly green fish, who appears commonly throughout the series, but in other episodes, the name "Old Man Jenkins" has been used for completely different characters, such as a green fish in a wheel chair and a blue fish in overalls. They are always elderly fish.
The reason for so many different versions of Old Man Jenkins has never been explained. However, there could just be different elderly fish with the name Old Man Jenkins, because Jenkins is a common name.
One of these Jenkins possibly died in "Friend or Foe" due to K.O. food poisoning.
This character's name is commonly used for gags in the show. This may also explain why there are different versions of Old Man Jenkins.
It is suggested that there may be more than one Old Man Jenkins as there are more than one King Neptune.