I think what looks like ageism is often not (but probably sometimes is).

It's more that, for most business programming roles, experience stops being cumulative at some point. For example, for a Java developer, 5-7 years is probably about where experience tops out, and beyond that it doesn't really help if you have 15 years experience in Java, or you have experience in Perl before you did Java, etc.

Cumulative experience of 10, 20, 30 years is really only useful for highly specialized roles (writing compilers, database engines, operating systems), management, etc.

I would worry less about ageism, and more about working on a track where experience is valued.