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.
[0] https://www.scientificamerican.com/article/at-what-age-does-....