Comment by Beltalowda

Also: you're learning the same thing you already learned several times before, except slightly different this time. Things are no longer as exciting as when you learned it the first time, and especially if $new_thing isn't a fundamental improvement over $old_thing, or even worse (in your view, whether that view is correct is another matter) it just becomes a slog.

I found that excitement/motivation is pretty important in actually learning new things. I'm close to 40 now and don't find it's harder to pick up new things when I'm motivated. If anything, I find it easier as I have a broader background knowledge and spend my time more effectively – I didn't believe my teachers when they told me that taking notes helps you remember stuff but they were right and I was a stubborn idiot. All of that offsets the undoubtedly decreased ability of my brain compared to when I was 21. It's just that I've done a lot of things before and find it hard to motivate myself for "$new_thing that's fundamentally just like $old_thing building $new_app that's not really any different from $old_app".