My company has had this contractor who is now 40 and it is impressive to see how productive he is. Moreover, as he has developed a great part of the infrastructure he has managed to make himself extremely valuable and hard to replace.
This is quite a feat in my company and in the industry in general as a lot of firms do not want to keep contractors for more than 2 years. He's been at my firm for about 10 years now; 10 years of contracting at I am sure £800+ per day! But I know that behind those high numbers there is some hard work and constant self-marketing.
The career yellow belts (1.2-1.3; 4th kyu) only get more set in their belief in their own rightness. They also tend to fall out of date, despite their expertise in the local maximum they've conquered. Also, since they tend to drive out the 1.4+, they aren't surrounded by better people.
There seems to be a bifurcation that happens very early on, based on a person's first few programming jobs as well as access to non-professional programming resources. Either you plateau as a yellow belt (1.2-1.3) or you start climbing, and that climb takes a long time (a 2.0 under age 30 would be very rare). I've worked about 3000 hours per year for seven years and that got me from ~0.8 to 1.8, but that's also with a lot of early exposure to programming (QBasic, Java in school, applied math research).
The shame of it is that some people conflate the career yellow belts (Expert Experts) with the older people who keep getting better each year.
You probably should make a link to that page on ycombinator for discussion. Well... first cut it down a bit so that it is shorter. (we live in the age of tldr :-p). But I think it was a good read.
In my personal experience (I'm 36) I get better with every line of code I write. I'm a far better programmer now than I was 10 years ago.