I suspect there are a couple of reasons - especially in the newer/startup space: * Salary. Older employees are more experienced, but, just as importantly, they've already job-hopped a bit, so their base is a lot higher. And they've probably a lot more personal financial responsibilities. All told, they're unlikely to go for the same salary as a new grad. * Interview process. Anecdotal experience, but, the interview p…
It isn't just job hopping. Even if you ignore promotions to a new title, many workplaces give out yearly salary adjustments that are a bit higher than inflation. Compound interest over 20 years adds up.
Note that I'm well past 40, so I'm in this boat. There are people with a very similar skill set to mine that are less expensive, mostly because they are younger.