I started to get pain in my right hand in my mid-20s (early 40s now) when I was spending a full working day programming and then spending most of my time playing fps games on my PC in the evening using KB+M. It progressed to the point that I would get stabbing pains on the top of my hand whenever I right-clicked my mouse.
After a few weeks of trying different layouts and avoiding my mouse as much as possible, I found the source of my issue was not my hand but a nerve in my elbow which was being crushed when using chair arm rests. I was also sleeping with my arms folded up on my chest which kept the nerve under tension when I was sleeping, preventing it from fully healing. I now avoid arm rests whenever possible and sleep with my arms to the side. The pain disappearred after about a week and I've been pain free ever since. I've also naturally reduced the amount of typing/mousing I do as I've got older and my career has progressed.