I'm about to hit 40 as well, and I completely identify with the pressure to keep up to date with new languages and frameworks, with the fact that I've lost memory of many things I haven't used recently, and that despite the proliferation of the new new thing, there really aren't that many new ideas out there.

But the thing is, I really like that I must always be learning. I just have that kind of brain that is attracted to learning new things, so for me this career has always been a natural fit.

In the last few years I learnt Meteor.js and built an issue tracker from scratch; I made several attempts at gaming projects using C++, C#, Swift, and others; I played Microcorruption and Cryptopals and Starfighter to learn a bunch about assembly, reverse engineering, crypto and security; I learnt Go and built a compiler with it; and right now my attention is moving towards lua to do some pico8 games. I did all this on the side of my boring corporate java developer job, and for no other reason than I wanted to learn new things. (OK, maybe I had big dreams of a startup with the issue tracker, but the others were purely for fun.)

I'm probably never going to be well known for any of those things, and I really haven't built up the chops to be considered an expert in any of them. But I'm content being a dilettante. Perhaps one day I'll get exhausted of exploring new things, but until then it's just fun to just dabble in whatever takes my fancy.