> I think ageism (in software) is largely a problem in startup culture.

Yes and no. Larger companies don't like hiring older employees because they can get more bang for their buck by hiring young(er) people who are willing to stick with the company for life. Being an older programmer who has been at the same company for 30+ years isn't proof that the company doesn't engage in ageism. It also means they're unaware of the current state of the hiring market and how brutal it's become over the past 10 years.