I would have to think a really long time to come up with an answer to “describe a hard problem you’ve solved” at all. I might have to consult notes. After that, to make it an interview-friendly narrative, I’d have to fill in a lot of details with plausible speculation because god knows I don’t actually remember (and I don’t really trust memory very much, anyway)

The joys of age plus extremely-poor autobiographical memory.

It doesn’t help that what I think of as actually hard, day to day, is shit like documentation from FAANG companies lying to me and wasting a whole fucking day of my time, terrible error messages, mismanagement of upstream projects, bureaucracy making a two-day task take four weeks, and, in some common ecosystems, awful design of core tools and major libraries, or horribly wasteful library churn. “Hard problems” are nothing next to bullshit problems.

Yes you are certainly right that it will be much harder the more years of experience you have.

That said, I think that someone could remember a few things for past projects when preparing for an interview. Of course doing that on the spot might be very hard.