I'll throw my anecdotal experience in life thus far in the ring. I turned 30 this year and I haven't really ever experienced anything more than mild stress, and even that is fairly infrequent (once or twice a year at most, and always because there is something that a reasonable person would say is worth stressing over).

I have been this way my entire life and as far as I'm aware it didn't come about from some life experience, I think I was just born like this. My mother used to laugh that everything was "like water off a duck's back" to me.

All throughout school and University I never once was stressed about any piece of assessment or any exams. There were a few I was woefully unprepared for and knew I would likely fail, but it never really bothered me. It wasn't that I didn't care, I just never saw the point in worrying as worrying is never going to change anything.

My wife is the opposite. She stresses about everything, many things that are completely out of her control. I have learnt a lot about how to support someone in that frame of mind, even when I don't understand why they are stressed.

This does have downsides. Many people, especially in professional contexts, mistake my stress free and easygoing nature as laziness. I also have relatively low empathy, though I'm not sure if that's a side effect, cause, or completely unrelated.