I collect old bping and camping stoves, having started acquiring them the last 4 years almost, so the last couple years I get out more than I used to, especially this last year. If its Saturday, I'm gone in the afternoon if not before, either bping or car camping, having to wait sometimes until then to pick up a stove or part at the Post Office, since they are open on Saturday 12-1. So being out so much, you are aware of the temp changes and much shorter days.
The last month or so now, its been in the 20's F by morning, so I've brought my WM Apache, 15F bag and my usual silk liner. I used an old Sears bag I got back in the 60's on a car camp trip a few weeks ago, and I got by I'll have to say, draping my old Golite Coal jacket over me in the early hours of the AM. Now I've gotten by in the Summer in July or August with my WM Caribou 35F bag when temps have hit 28-30F here in the Sierra, suffering very little if any from those temps during the short nights. I noticed especially this Fall, that my 15F bag sure feels good when temps hit the 20F's. I also turned 60 recently and am about 160lbs. and 6' tall. Age or long nights? I'm thinking more the long nights at colder temps, turning colder early, versus summers short night duration of cold temps, slowly getting colder. My bags are in good shape, the Caribou having been washed last winter and the Apache has few hours, so it has never been washed, being maybe 6 years old.