When I was a young 20 something starting out in the bay (20 years ago) I ran across quite a few older engineers (some of them in their 50's).
All the best things I know came from them.
I remember at one point one of them handed me a spiral bound book of the papers from an early 90s (or late 80s) unix conference. Papers, actual papers, from a conference, BOUND (pre internet right there) --- there were a lot of interesting insights that came from that book that still impact me to this very day.
Could be plenty of noobish 40 year olds out there.
cough
I mean, a lot of times this is coded in the job descriptions. "rockstar ninja" "10x engineer" or similar words generally mean they are unable to pay experienced people wages and are looking for someone new to do the work of an experienced person.
A lot of people think this is bad, as it almost always implies overwork, but I have mixed feelings. My first programming job? I was 17, no education, and I was hired as a "Staff Programmer" but only paid slightly under $40K (this was in '97, so while that was great money for a 17 year old, that was... not market rate for an experienced engineer, much less a "staff" level title.)
In a lot of ways it was a great experience for me. My shitty C code ran on production and in turns vastly improved and degraded user experiences for literally millions of people. For a company that wants to 'roll the dice' - I was probably a good deal, 'cause I did some pretty amazing things. (in both directions, really... but it did work out well sometimes?)
(I can't find the quote right now... by Vixie; something about how many CERT notices he was responsible for, and how it showed that in his youth, he could code, but he didn't learn to manage risk until later.)
But, I mean, my experience of the valley (and SF is not silicon valley, but it is... adjacent.) is that startups hiring young people can partly be explained by the phenomena that startups aren't willing to pay market rate for someone with experience; even at my age, I see pressure to move to management.
If something bad happened and I had to work at a startup again, one of the big tells I'd look for would be the number of older engineers that are working as direct hires (not contractors); This is usually a good indicator that they are willing to pay reasonably.