Maybe one of the fields that has the most "ageism" is programming and software engineering jobs.
There were multiple times I hear of the CEO or SVP at the final approval stage: "we want to hire a more junior person", and rejected the candidate and the 10 hours of interviews by 6 tech people and 3 manager level people.
Some say, before age 35, a person is most creative and productive, and it is all about productivity.
In reality, when you are 25 or 28, you are more likely to stay up working till 3am, sleep, and wake up at 9am and continue to work. However, most companies claim they want 40 hours per week and work life balance, so they won't burn somebody out in 1.5 years and then have to dump them.
Also, programming and software engineering isn't like painter or poet or novelist. They don't need that much creativity. They are more like engineers building roads or bridges, where experience is important too.
One other reason I can think of is, if the manager is 32, he wouldn't want to hire somebody 38 or 45, because then that person appears more like a manager than he or she is. (and that a 28 year old is more like to listen to the 32, and a 45 year old is probably less likely).
And just because they want to hire somebody 35 or under, isn't it ageism and therefore, discrimination by age, and therefore illegal in the US?
I even talked to a lawyer that says, "if we sue, that means these cases are flying all over the sky" (and therefore it is like people quietly accept them).
It also appears that the CEO or SVP are bold enough to even at the final offer approval stage, say "rejected. We want a more junior person."