I've got tires from the 50's on a few balloon tire bikes that are still good. I don't see why a set of Schwinn tires couldn't survive from '74.
If they were kept out of the sun and stored where they at least somewhat protected they could still be fine.
I've bought more than a dozen new old stock bikes over the past 20 years, only one came out of the box with tires that were degraded, and those were only showing very minor cracks in the gumwalls. I've got several pair of vintage IRC and CST tires that I wrapped up and bagged in the mid to late 80's for future use that are still like brand new as well. I had gotten a deal on them and packed them away by wrapping each tire with plastic and bagging them in pairs. I vacuumed out the air, and boxed them up. I used a pair a few months ago they're like brand new. I think UV light and avoiding exposing them to ozone from running motors and such is a big key to getting rubber to last.