This has nothing to do with age. In most people's case swim lanes are set early in life. If you weren't good enough at 23, unless some major incident in your life changed you, chances are you will be the same at 40.
This has largely been a problem- most people feel with this popular phenomenon called 'Nerd culture'. Some people have it all figured way early in life, others take long, many never. This fact alone counts for different starting points in life, overall seriousness, and the net time available to make progress. Also if you started out as a Nerd earlier, you are more curious, and more eager to learn, change and adapt to things. Than people who start out later, who just wanted a job to provide for their family, and are in purely for 9 - 5 kind of a job, those people will routinely find themselves stuck in the wrong job- As they have to spend bulk of their time learning things. While they might want to spend that time on recreation.
This whole thing about learning and exploring things just for the heck of it doesn't come naturally to most people.
From my own childhood I remember a bunch of bullies from my lane, routinely taunting and beating up kids. One of them grew up to his mid-20s and woke up to the financial realities of life. He then went to college and did his engineering. But that was 8 years after I had finished my degree, and had a neat 9 - 10 years experience compared to him. I once met him in a bus and he came across as a totally changed person, but he still struggles. Largely because the impulse to do things is different, he just wants a job, and beyond that bulk of his early life throughout childhood, teenage and early adulthood were spent doing unproductive stuff. So there is just no desire or curiosity, or even the mental tools required to sustain a continuous learning life long marathon campaign.
I was mostly trying to provide a counterbalance to the "older is always better" opinion I've seen as well. I was lucky enough early in my career to have a myriad of stellar mentors - both old and young, and learned to simply listen to what folks have to say.
As I get older I do understand how hard it is to keep up with the latest thing in tech, while also handling other responsibilities in life. I almost always tend towards the grumpy guy ranting about the latest fad, so this comment is largely out of character!
I thought I'd toss it in my earlier comment - but it's also selection bias at play here. The guys in their 40's looking for work are likely of a different type than those in their 20's. The stellar candidates who have decades of career behind them typically have a strong social network that places them before they even look at a job board. This is an area where I feel ageism is very real, and kind of terrifies me a little should I find myself in that position.
Older isn't always better, simply because older isn't a separate species or class of humans. Put simply young people eventually become old. If you had major human enterprise issues while you were young, chances are those will remain when you get old.
This extends to other things too.
Why do you think retirement is so hard for people who didn't save well enough in 20s and 30s? Nobody can wake up one day, and run a ultra marathon. Processes for making something like this happen start way early in life.
Just as the majority of 23 year olds are what you are imagining (inexperienced, know-it-alls, chasing the latest fad, etc.) I would say the majority of 40 year old programmers are just as useless in the other direction.
Maybe not on HN where everyone is above average, but in the flyover states it's absolutely the case. Exceedingly few 40+ year old programmers around these parts I'd describe as holding a relevant modern skillset or holding any interest whatsoever in updating their knowledge or trying to stay competitive. It's almost universal derision over the "flavor of the month" until 10-15 years down the road when they realized flavor of the month turned into Linux and all they have left is bitterness over being left behind.
Just as you have the hiring skills to weed through the huge cruft of useless candidates of your preferred type, some hiring managers hold the skills to wade through the cruft of young hipster bros and find the gems as well. The 80/20 rule typically applies to all things in life.
So put another way - in the end, I'd say it all roughly evens out in my experience. I think age is an extremely poor metric for productivity, and it really depends on the specific individual.
I say all this as someone much closer to 40 than 20.