I’m 40 and it’s less that I can’t learn new ideas and it’s more that I have fairly settled ideas about how the world works and it’s hard for me to fully integrate new concepts without relearning a lot of other things at the same time.
I can learn new things as well as I could before, it just takes a bit of effort to clear space for it, as it were.
That you used the word "relearn" responding to a comment that mentions docker kinda hits it exactly for me: it's hard to get excited about a new way to do the same old thing, whereas when younger you don't yet have enough understanding/experience for it to have reached "same old thing" status.
I can learn new things as well as I could before, it just takes a bit of effort to clear space for it, as it were.