For me it's usually more the quality of the film/print/color/etc. Most 70s pics have a certain color/tone/hue/saturation that says '70s'. 60s too. And so on. There may be easier determinations between 1920s and 1930s pics, perhaps, but not quickly to my eye.

What's interesting about these is that... they're candid. Most 1800s photos were people explicitly sitting for a photo. Not smiling. Sitting, posing, very grim. Many of these pics showed people being natural - smiling, chatting to each other, etc. Normal human stuff.