You can't teach an old dog new tricks

Origin

Imagine a 16th-century English farmer, exasperated with his aging hunting hound. The dog, once agile and eager, now stubbornly stuck to its old methods, ignoring any attempt to introduce new commands or techniques for flushing game. This common frustration, the difficulty of altering ingrained habits in older animals, crystallized into the proverb. While early forms like "It is hard to make an old dog stoop" appeared in John Heywood's 1546 collection of proverbs, it was the keen observation of persistent animal behavior that truly gave birth to the phrase. It reflected a universal truth: as creatures age, their established routines become deeply set, making the adoption of novel behaviors a challenging, if not impossible, endeavor.