Used bikes that old are case-by-case.

"Barn finds" are more likely to have rotted tires and brake pads, calcified grease in the bearings (bottom bracket, hubs, headset, pedals), and so on. But the metal might be in better shape if it's been indoors and not getting wear. While a bike that got ridden all the while might be pretty clapped out or have wildly different components due to replacement, but it's but maintained and ready to ride

I got a really nice Motobecane recently and it seemed like it was in marvelous shape, but it had cracked tires, brake pads like charcoal briquettes, and some pitting in the front hub cones just from getting water in there that sat a while. It's docked right now because the chain broke for no reason I could parse.

Location matters too. Here in California's elbow, nice old bikes are plentiful, due to both bike culture and dry climate, but in places with neither, good luck