[jacknews]

It's not just about salary.

Of course the assumption is that they have to pay more for more experience.

But it's also about perceived power and control. Younger workers are seen as more compliant, and many managers do not feel comfortable hiring underlings who are older.

[nickd2001]

Agreed, there are indeed managers for whom sadly its about control and compliance. However, there are also nice managers who just might feel mildly insecure having a more experienced person as a subordinate. Perhaps the onus is on the older dev to be sensitive to that and find a way to make the manager feel comfortable with sometimes being "managed up"? To know that the experienced person is a helpful ally not someone who'd undermine them. It may be something to be direct about in interviews. Obviously the interviewer is prohibited from raising the subject due to discrimination laws, but there's nothing to stop an applicant saying that they really don't mind having a boss that's a lot younger, and that they just aim to be of help.