Ask HN: Is there any logic behind ageism?

I'd wager some money that, on average, young people are both more vulnerable (w.r.t unemployment) and less experienced in negotiation, leading to equal or greater value for employers at lower cost.

I know with one employer I worked for. They frequently noted two reasons why they preferred younger technical people (in casual conversation). One, Younger hires are just starting out their career so work cheaper, longer as well as cheaper benefits (younger age brackets and no or just starting family). two, They seemed to think fresh grads or candidates that were younger had fresher knowledge and were more likely to have new ideas and be familiar with newer tech or methods. Number one is probably correct in general, though I would say potential experience and maturity is worth quite a bit. Number two, is a misperception and in my experience age is an unreliable indicator for the freshness of ideas, or application of knowledge.

To the usual list of reasons, I'll add one less often discussed: it is awkward for a boss to manage someone older than themselves. A lot of social authority depends on age and experience, and having older staff makes that story more difficult.

The upside: if you are older and running your own business, then you move from being an employee to a vendor. Social graces remain intact.

If you are an employer and can look past the irrational social biases, you can find a lot of great employees no one else will hire.

Old people cost more. Not just benefits (as gabeeg noted), but salary as well. If you don't need 30 years of experience, why pay for it?