Uh, isn't that rather much the point? Once a young person learns all this stuff from experience, they've become an old person.
You cannot magically learn everything out of sheer willpower.
Additionally, a few things are impossible to learn without time: the "Judgement" section in the article. You can certainly theoretically understand the meaning of it, but to experience it and have a deep, intimate understanding of it you have to go through the process of writing something and supporting it.
So it's not a matter of correlation / causation, it's a matter of the two concepts being two sides of the same coin.
You are absolutely correct that experience takes time and that the type of experience that the author speaks of comes from writing something and supporting it.
What I am saying, is that you can choose to write different types of things and support them, without having to become 40+ years old. Not out of "sheer willpower" either; you just have to choose to write different types of things and support them. Yes, you will be older than you were before you started. But... to get that experience you do not need to reach age 40.
If anything, simply getting older is not a guarantee that you will get that experience. Suppose you only program CRUD apps for 20 years with the exact same stack every time. There's only so many times you can write the same sort of app over and over again before you get diminishing returns on that kind of experience. The author knows this implicitly, but assumes from his own experience that other 40+ year old programmers will have as wide a range of experience as he does.
That's really the specific premise of his that I'm questioning: that being an old programmer automatically means you have a wide range of useful experience that makes you better.
And it's not just about having several projects under your belt. This is making the fundamental mistake that software development is a pretty intellectual pursuit, when in reality the social aspect is far more important.
There is living through the cyclical zeitgeist. There is the experience that only comes from having been through things when there were new, and having done this several times. There is no way for a young person to know what it was like to start writing JavaScript when it was first invented, or to have made one of the earliest AJAX apps when XmlHTTPRequest was first invented. That's not an argument that the old ways are better than the new, it's an argument that we know deep down why the new ways are better than the old (and what lessons the new ones could learn, too).
There is the super powers that come from having spent a long time in two or more completely different fields. There is domain knowledge that does not come cheaply, and to gain such expensive knowledge in more than one field is so rare.
I don't think the author is at all arguing that all 40+ developers are great. He's arguing to stop universally dismissing older programmers.
In the last 20 years I've gone from writing desktop applications using RAD tools, to websites using no tools, to web apps using frameworks, to mobile apps using IDE's, to mobile apps using web app tools.
In the same period we've seen the rise and fall of Object Orientation, the fall and rise of Imperative/Structured Programming, and the burgeoning new age of Functional and Reactive/Declarative Programming.
Also Agile came and devolved back to Scrum Only, and waterfall never stopped being the default.
All version control systems got converted to Git.
SQL fell out of fashion for being too complex and then everyone worked out why all that complexity was needed.
You can't be in this industry for 20 years and work on one stack writing the same sort of applications ;) Maybe five years, tops.