He just means: by this age you've probably found your preferred title and level, unless you want to rise to more C-level / executive positions, which are rarer in any case and most folks don't want.
This is definitely more charitable, but isn't this already the case now? It seems he's saying past your mid 30's you'd no longer be viable as a software engineer. That's never been the case, and I'm not sure why it would now suddenly be the case.
Even clearer, if you don't adapt to the changes taking place in the field there might not be a future for you. Its not about age, it is about attitude and flexibility (which are, admittedly, issues when getting older).
In other words, if you want to continue stubbornly typing out code by hand, the person right over there has already mastered agentic tooling and is doing vastly more than you, more quickly, and with greater precision, and will simply be a more fit candidate to hire. Roles for this type of legacy stubborn personality will be less and less, and you will age out as part of the old school.
I see what you're getting at, but if it's not about age, why use an age related analogy? I probably should have amended my first statement in this thread is that it sounds ageist, if even implying that the people who will refuse to adapt will be older. This day is already here, people are already adapting to this. He seemed to frame it as the current young 20's career people will have this limited timeframe of productivity.
Age analogy is fine because unlike few who are deep into technology and latest changes in the field (which btw are over represented on this site) for most IT developers age => experience without actually improving skills.
As I interview lot of people for typical Enterprise IT jobs even at 20 years of experience they do seem to not know much beyond what they learned in first few years.