In 1735 Henry Woodfall, a printer's apprentice, wrote a ballad titled "The joys of love never forgot: a song" about a happily married elderly couple. His inspiration for those characters was his own boss John Darby and his wife Joan:
"Old Darby, with Joan by his side,
You've often regarded with wonder:
He's dropsical, she is sore-eyed,
Yet they're never happy asunder ..."
He wrote this poem after Darby's death. This poem in turn became an inspiration for follow-up poems and eventually Darby and Joan became a metaphor. In the UK, clubs for old people are still called Darby and Joan clubs. Below is a pic of D&J figurines.