Donkey’s years

Q From Jess Paxton in the USA: Is there a story behind the phrase donkey’s years?

A It’s a pun on donkey’s ears, they being long. The phrase, meaning a long time, is chiefly in British use, though known in the USA and elsewhere, and was first recorded in 1916 as donkey’s ears (which is why we’re sure about the punning origin). Within ten years or so it is recorded in the modern form. The idea was supported by the belief that donkeys did in fact live a very long time.