Have you found it harder or easier to write thoughtful+intelligent posts as you've grown older?...

As a very young person myself compared to some others (just 23), I can't say I have much insight into this issue. But I'll shed what light I may.

I can say that over the last 10 years (I've always been a heavy reader & writer), my technical skills haven't improved very much from my perspective. My prose style used to be more show-offy and complex than it is now; I'm not sure what lesson to draw from that. (It is not as if my WM or IQ plummeted when I turned 19; if anything, with n-backing, it is probably higher.)

What has improved is my ability to think up real-world examples, and ability to add citations. My essays used to be full of holes and occasional non sequiturs, but as I read more and more and the Internet became more useful, they started getting better and I began to be able to offer useful comments.

For example, I first emailed SL4 in October 2004 with a useless contribution, and most of my early emails are marginal comments I would neither up nor downvote if I saw them as comments here. And in another example, I thought up a really good insurance idea, wrote a little essay on it, and years later read of something identical; I had reinvented a standard form of insurance. These days, I think I am doing better; some things on gwern.net I am not even ashamed of.

Additionally, do old authors write any less well than younger authors? Especially authors who are very old? Jacques Barzun, in particular, wrote his last book at age 99. But it seems that most people have stopped producing by that age. It is said that Hans Bethe was the only nonagenarian physicist who produced a top-calibre publication in his 90s, for example. Of course, many of these effects are average, and beating the average is pretty easy if we're motivated enough to do it (simply because many others aren't motivated, and also because intellectual stimulation helps stave off brain shrinkage). But even in the end, beating the average may not be enough to prevent decline.

Need to break out what you mean by writing. Technically, I'd say writers usually only improve. Gene Wolfe's prose seems to be getting smoother and subtler by the decade.

Idea-wise, they can often go downhill. Take Gene Wolfe again. He's pretty prolific and only has grown in critical acclaim. But never the less, the books the fans and critics keep coming back to are some he wrote 30 or 40 years ago, The Book of the New Sun. The Book of the Long Sun and The Book of the Short Sun are its stylistic peers, but they get only a small fraction of the attention & love of New Sun. There's no obvious reason why they aren't more popular. They just aren't. Some magic is missing. (And on some respects, they are distinctly inferior: his world-building is growing outdated, his politics have grown increasingly obtrusive & shrilly conservative, and his puzzles obtuse and difficult to solve. On urth.net, the consensus is that we don't understand An Evil Guest or Pirate Freedom at all, and with The Sorcerer's House, the puzzles seem simplistic enough that we're not sure we're missing the forest for the trees.)

Or more obviously, there's Frank Herbert. Dune and its universe marches on, apparently untouched by time. But when was the last time you even saw any of his other novels like The Dosadi Experiment or Dragon in the Sea discussed? Only fans read those, and I can attest that even we don't read them often or deeply; as well, there seems to be a real trend toward them getting worse.