It might be referring to job role rather than "age." A Staff Engineer is supposed to be inspiring (both up and down,) so doing quick tests fits the bill, and that's something LLMs are great at supporting. Mid-level is mostly about delivering reliably.

The question is if LLMs/tools could drag mid-level earlier into senior level, or if it's a phase one has to go through. Ultimately, the tangible promise of LLMs is to push the entire timeline up, so that junior tasks are automated and you go in and control the LLMs. Expectations on what it means to be a software engineer are sure to change, at some point. (I like the software craft as-is, but fact is most of our lumber is straighter now that we have automatic saws. And I've never heard a carpenter pleading to split a log manually.)