The blog post isn't about self-reported cognitive ability getting stronger (or not declining). It's about the job marketplace.
Maybe it's instructive to compare/contrast different professional careers that depend on mental abilities and how age affects marketability:
- computer programming : oft-reported industry bias against age 40+ and active recruitment (especially by software companies) for 20-somethings.
- corporate executive manager (e.g. CEO, COO, CFO, etc) : age 40+ is typically the target hire age. Being a younger 20-something actually works against being hired for these roles. (To be CEO at age 20, you'd have to be the founder of a startup.)
- surgeon : age 40+ doesn't seem to bother patients. They may rather have a 55-year old do their heart bypass rather than a young 30-year old just out of residency.
Why is experience valued for outside hires of CEOs but not as much for programmers? In other words, a 55-year old programmer would have 35 more years of experience than a 20-year old but that "extra 35 years of programming" doesn't seem to be valued as much as the "extra 35 years of managing" that a 55-year CEO candidate has. Why?
I have pet theories on that but I'd rather hear what others think.