I'll add a few points as an over-40 developer/recruiter currently working for a startup:
* By 40, they've probably already done the startup thing once or thrice, and many have been there, done that. They are also less motivated by startup stock options, having seen them evaporate personally.
* By 40, they are more likely part of a family unit that values financial stability.
* By 40, they are at their salary cap. They are simply more expensive. They've also been around the block enough to have connections and to play the regotiating game harder.
Overall, I think the strongest factor is simply the demographics you mentioned. There weren't that many folks that graduated in CS until the 2000's. Also, many of the folks that are around my age have switched careers away from development.
* By 40, they've probably already done the startup thing once or thrice, and many have been there, done that. They are also less motivated by startup stock options, having seen them evaporate personally.
* By 40, they are more likely part of a family unit that values financial stability.
* By 40, they are at their salary cap. They are simply more expensive. They've also been around the block enough to have connections and to play the regotiating game harder.
Overall, I think the strongest factor is simply the demographics you mentioned. There weren't that many folks that graduated in CS until the 2000's. Also, many of the folks that are around my age have switched careers away from development.