Younger ones have the benefit that their skills are still likely to be relevant at a larger subset of the employer pool.
Also, in my experience, stagnation can run in the other direction, temporally. I encounter many young people who won't even learn about, much less consider, using a tool like 'make', simply based on the assumption that newer seemingly similar tools must be better. Sometimes the old ways really are superior for the task at hand.
Most of the successful tech people I know that are 50+ never overstayed their welcome at companies where they stopped learning, or they were always being recruited by other firms and moved around. Recruiters generally aren't knocking on doors of someone who has been at the same company for 20 years, and that's for a variety of reasons.
Not the way you're describing it. However, there's a pretty common correlation between age and relevance of skillset, which is a direct consequence of stagnation over many years.
The point is that someone saying "old == stagnant" may actually be thinking of it the way I'm describing, which has more validity.
Younger ones can have all the latest Javascript library experience they want but they still don’t have enough obvious experience for engineering mastery that’s vital to being a highly-productive independent contributor or tech lead.
Read it again. It's referring to older stagnant people, not all older people. The longer someone has been stagnant, the more out of date their skills become.
Because that is the case generally speaking. People become more resistant to change as they get older. They become more conservative, their openness trait decreases.
That last sentence sounded a bit cynical - So anyway, the main trick with being older in tech is not being open but remaining enthusiastic (most days)
Suppose my boss asks me to find a good Thai restaurant for lunch. If I'm new to the area, I'll be very "open" and spend a lot of time looking into the possibilities, etc. If I have a lot of experience eating at Thai places in town, I'll be less open. I already know the best one out of a substantial set, and it's less likely that I'll find a better one without a lot of work. Call it stagnation, or call it being careful with the company's money.
If you weren't stagnant when you were young, you're not going to be stagnant when you're old.