I was noticeably older than most of the people in my team in Amazon, and I was never shitting bricks[1]. The ones not reaching performance targets were always crystal clear to _everyone_. If you can't spot the underachievers, maybe that's the time to worry.
Despite how detailed the Amazon interview process is, people slip through the gaps and you end up with someone who somehow can barely write functional code, let alone maintainable code. Or you find the person is capable of writing code, just maybe not today, or tomorrow. Is the day after good with you? There are much more interesting things to be doing on-line instead of writing code.
[1] I was reminiscing with a co-worker in the kitchen one lunchtime about the respective things we'd had to do to deal with the Y2K bug at the places we'd been working at. Another talented and very capable developer co-worker overheard us and commented "I'd never even touched a computer before, back then."