The issue is whether (or not) you have ‘brown’ fat. This is fat with lots of mitochondria that are quite active in creating heat. (on top of that, there are some interesting inter racial differences in the mitochondrial ATP phosphorolation mechanism —so higher latitude peoples turn more energy into heat body-wide).

Kids are born with that fat, lose it as they get older. (makes sense given the changing mass/surface ratio). On top of that, I suspect it’s a “use it or lose it” kind of thing.

Recently, there’s been some work indicating that repeated prolonged cold exposure can recruit some ‘white’ (simple lipid storage) fat cells into something very like brown.

Also, if you look at what Wim Hoff has been doing, it looks as though voluntarily pushing blood pH towards alkalosis through hyperventilation can decouple the pain response from extreme cold exposure. Interesting times…