It’s hard to give a hard-and-fast rule on exactly when a tire is ‘too old’ (as opposed to worn out) and needs to be replaced.
For a 40-year-old original tire, I’d say if they hold air: Ride them as far as you’re willing to walk home, and as fast as you’re willing to crash.

A lot of it depends on storage conditions, and whether it sees any use. I had the 8-year old skinwall Continentals on my Cannondale come apart during a ride, I was storing the bike on the porches and patios of various apartments; the Ritcheys on there now are in far better shape, despite pushing 15 years old, but it lives in the garage, now.