Hurhurhurhur
I liked this movie, but this is not an original joke. It is at least as old as The Toy with Richard Pryor.
Nobody gets to be a cowboy forever.
The joke is at least 100 years old. I certainly remember it going around the schoolyard more than 50 years ago, and it was old then.
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Old joke or not. It is funny. Double entendre, unintentional pun, and (knowing) response to said unintended pun, all rolled into one.