I wouldn't take a chance with old tires. Even if they look okay. Too much can go wrong if a tire fails. You can buy decent new tires for around $20. Continental Ultra Sport II or III are excellent values, although they're notoriously tight fitting and it's handy to keep a bead jack around, like the one sold by Kool Stop, just to mount those tires.
Ozone and other environmental conditions (including UV) are a big factor in the longevity of rubber and plastics.
I've seen indoor ozone generators destroy rubber and plastics. Sometimes the generators were deliberately installed, those "ionic air cleaner" things people used to plug in around the house, thinking it would only "clean the air." Those ionic air cleaners destroyed items in my grandparents' home -- outlet plates, switch plates, coffee makers, even insulation around wiring, anything made of plastic near the ionic air cleaners. And it sprayed sticky dusty residue around every outlet where those things were plugged in.
Other times it's an old, defective appliance. About 10 years ago a neighbor, who smoked like a chimney, was complaining about her TV. I offered to take a look. When I tried to turn the set around to check the back, the back of the TV crumbled like potato chips. The insides were coated in a black, sticky goo, which was also sprayed on the wall behind the TV. I realized it was from her incessant smoking -- she probably smoked two packs a day and, as far as anyone could tell, didn't eat or drink much. She appeared to subsist almost entirely on tobacco.
I told her the old set was a safety hazard, and offered to give her one of my old but good Sony Trinitron portables if she'd let me discard her old TV. She agreed. I was concerned about a fire hazard that would affect others nearby.
Same thing can happen with many old electronics, including older laser printers.
So besides normal environmental hazards, there's generalized pollution that plagues almost every city, and localized pollution in our own homes, all of which can degrade rubber and plastics.