When was the Golden Age of fast food? (I mean fast food from chain restaurants like Burger King and McDonalds.)
It might vary by food type - burgers vs tacos vs chicken vs fish.
Burgers:
If we are emphasizing "fast", the Golden Age might be now. I'm imipressed with how fast burger orders are filled.
If we are emphasizing taste then I think the Golden Age was when burgers were grilled instead of microwaved and when fries were cooked in oil that had bacon grease in it - or whatever unhealthy thing McDonalds used for fries in the old days. I don't know the dates that define this period.
I seem to recall a time when "with onions" got you a substantial amount of onions. Nowadays, "with onions" means "only enough onions so that someone who gets onions by mistake will take them off instead of bringing the burger back". I think "with mayonnaise" was not a common option decades ago. However, if a place offered mayonnaise it was normal mayonnaise. Nowadays, they seem to put on some sort of liquid.
Tacos
Is there another national chain besides Taco Bell? I haven't eaten fast food tacos very often, so I don't know the history.
Chicken
Whenever the Golden Age was, they didn't use the oil that the Colonel uses today. I think it is canola based and it reminds me of linseed oil.
The chicken at burger places has the same tough texture as the chicken in frozen dinners. I haven't detected any change in it over the years.
Fish
I think the fish sandwiches at burger places now are better than they used to be decades ago. However, they are still somewhat tough, like the fish in frozen dinners. As to Long John Silvers, nowadays I only go to Long John's after enough time has passed for me to completely forget my last visit to Long John's. I can't remember how their fish was decades ago. Perhaps they've improved in recent years.