24 yo here, so I can't answer your question. BUT. From what I read in your post, you're not only asking if there is a future in tech, you're also saying you're afraid because you don't have as much energy as you used to.

I'd say energy has little to do with age. As every 40yo you probably are 20lbs overweight, you haven't done much sport in the last years, you probably don't manage your sleep as well as you should, same goes for nutrition, etc.

When is the last time you took a MOOC, a new formation, learned a new skill by yourself?

tldr your energy has more to do with your lifestyle than with your age. Gl. :)