Since I log my own hours I became full aware of this 6 hours "ceiling" in the long term, for a few days I could work for many more hours, but that would have a big price to pay in future productivity. In the beginning I felt bad, as if I was not able to produce as much as other people leading to a lot of anxiety. As I realized that my actual productivity was going up, compared to my older self and other peers, I embraced it.
Now I work 6 productive hours and to me that's 8 hours of work, my results are better month by month, my quality of life is better and my anxiety is more controlled. Everybody wins.
"Fluff" time is for sitting in meetings or writing reports, as work functions, or for just looking busy. I intentionally avoid doing real work that requires any focus or creativity in the fluff slot. After six hours of real work, any more time spent doing the same thing starts to become counterproductive. So just leave it for tomorrow.
I have been more productive while explicitly dedicating 2 hours a day to getting nothing useful done, than when trying to stay focused on one thing for 8 hours, and ending up goofing off, getting sidetracked, or procrastinating for some amount of time anyway.
That's the theory, anyway. My current employer doesn't keep my pipelines reliably filled. During the dry spells, I have a hard time expanding the fluff slot to more than two hours, and end up doing stuff like coding stupid CLI games, or vectorizing the company logo, or setting up a new vanilla VM that I might not ever use.
I can still code for longer than this, from time to time, but only if I already know exactly where I'm going. Even then it's playing with fire.
Maybe sober, but that's not the limit once you factor in the rampant use of amphetamine.
It’s true you can knock out a 10-20hr session in essentially one sitting, but it’s not a good habit to set for yourself or the industry
edit: https://www.theguardian.com/books/2008/dec/02/jack-kerouac-r... the "scroll"
Outside of extremely rare triage situations, this limit is true for all developers, regardless of age or quality of output. Beyond about the 6 hour mark, people can force themselves to sit in a chair and hit keys on a keyboard like the Swedish chef from the muppets, but it's pure management denial to believe those hours count as "productivity" and in fact when the defect rates start to get high and fatigue compounds and morale drops, it's really anti-productive.
Really, younger age is just usually correlated with fewer extracurricular responsibilities, i.e. home repair, health maintenance, domestic responsibilities, marriage, children, etc. These things tend to increase with age, making it a scheduling impossibility for older workers to sit in the chair at 8 pm going "hur duh dur" while slapping keys.