I wonder if diet can affect the adaptation as well. I grew up eating bland scandinavian food and could tolerate the cold to the point where I’d wear shorts and short-sleeved shirts all year round, go out in the snow barefoot and in shorts without discomfort (not for prolonged periods, obviously, just going out to check mail or get something from the family car), swim in water that only Russians thought was comfortable (actual anecdote, as a kid on some trip to the beach in the spring or fall the water was so cold that the only other people there who were swimming were some Russians, even my scandinavian family thought the water was too cold lol), etc, then as an adult went without AC in the south for over a decade and started living off extremely spicy food with lots of the hottest peppers I could get and now ~80+ is pleasantly warm, 90+ is tolerable in the shade with a fan going, and when it gets down to 70 I’m starting to be cold and dressing warmly or huddling under a blanket as my extremities start to go stiff and numb (well, that’s more around 60 and below, but still).