It seems that most countries with reasonably stable currencies are reluctant to demonetize obsolete issues, for fear it might instill uncertainty or even panic. In the U.S., any note dated from 1863 on is still potentially legal tender, except for older issues that were actually redeemed by their bearers and never re-issued. For example, in 1935 there was a fire at the U.S. Treasury building, and in the commotion, a bin of $1000 notes was spilled into the street. Passersby eagerly grabbed up what they could–but in the end they were all worthless, because they had all been previously redeemed and were no longer outstanding. But if some of those notes were still outstanding–whether in someone’s wallet or safe deposit box or wherever, they would still be good today. You wouldn’t be able to take it to the treasury and get gold coins for them, but you could spend them at a store or whatever.