> Hiring managers don't consider themselves ageist, but
> Hiring managers don't want to be ageist, but
I classify this into the "I'm not a racist, but..." bucket.
> Hiring managers (usually fixated on 'new tech') who fear diminished learning, adoption or performance capacity in older employees
This is the textbook definition of what ageism is.
Conclusion? They are ageists, plain as that. They may not consider themselves to be, or want to be, but they still are, because ageist is as ageist does, and it matters jack what appearances they want to keep or what they think or who they perceive in a mirror.
I won't address "performance capacity" directly since it's too broad and vague, but it is plausible that there is diminished learning as we age. Think about learning new spoken languages. There's evidence to back the idea up in that context [0]. At the same time a 40 year old will likely have a higher proficiency at their language(s) than a 20 year old. This analogy exaggerates the idea (the trade-off) but I don't see why it wouldn't apply to programming languages as well. And this isn't ageism.
You're ignoring that most "new" language problems have significant overlap with things already experienced, and there's only a minor translation issue, rather than a learning new things from scratch issue.
> Hiring managers don't want to be ageist, but
I classify this into the "I'm not a racist, but..." bucket.
> Hiring managers (usually fixated on 'new tech') who fear diminished learning, adoption or performance capacity in older employees
This is the textbook definition of what ageism is.
Conclusion? They are ageists, plain as that. They may not consider themselves to be, or want to be, but they still are, because ageist is as ageist does, and it matters jack what appearances they want to keep or what they think or who they perceive in a mirror.