I think they would need to be very old, close to a century old, to have any real effects. I only say this based on having no issues using lots of bullets that were 30,40, or 50 years old. And the only real issue I recall that would be somewhat related is WWI era brass that had weakened with age, though it becomes brittle as opposed to annealed.
Also remember that back 50 years ago there were no really tough bullets for the most part as we know them today. Bonded bullets were in their infancy, and the Partition was the main premium and they have a pretty soft front half. So someone used to using the excellent bullets we have today and expecting those types of results might be disappointed.
