For what it's worth, I did the same self-experiment at various times in my 20s and early 30s. I got the same result: none of the headaches, grumpiness, lack of

For what it's worth, I did the same self-experiment at various times in my 20s and early 30s. I got the same result: none of the headaches, grumpiness, lack of focus, etc., which I'd heard people experience going through caffeine withdrawal. I came to the same conclusion as you: in the same way that I don't notice a significant uptick in energy when I drink coffee, maybe I also don't get the withdrawal when I stop. Maybe some people are just weird like that? Then I did the same thing in my early 40s, and the headaches started on the second day. It was pretty bad actually, and I understood what people meant when they talked about an actual, physical withdrawal period. So, now I think it's just metabolism (or some other characteristic that changes with age), rather than a blanket immunity to the effects.