I remember doing a winter brevet in a warmer climate with temperatures at the start in the low 40's and low 60's expected during the day.
A couple locals were in ski parkas and everyone, except me, was dressed like it was 15F. I had wool leg warmersand synthetic armwarmers. SS baselayer, jersey, and reflective gilet. I wore thin wool gloves. I really regretted the base layer once it got hot out.
The bottomline, it is very individual and cold adaptation plays a role. I remember doing a 200K in December or January in PA and one of the riders was in sandals with no socks. He explained he only put socks on when the temps were below freezing. It was 35F and I got sleet coming down Jugtown Mountain. To each his own, I suppose. I usually carry a light down jacket in my bag. Someone stole it one day when I took too long of a break at a cafe on a cold March ride. I need a new one that fits, they take so little space and can be a lifesaver when fixing a flat.