Agism is real, but probably unintentional. By definition, most people hire subordinates, and in their mind they picture someone some number of years their junior. When an older person applies, they just don't fit the image they have created in their mind (assuming they don't feel outright threatened).

It might even be that agism has the opposite effect once you are established in an organization. If you are qualified, experienced and engaged, you might be first in line for promotions because then you fit the image.

The best advice to fight agism is probably to use your network. If you've been around the industry for many years and made friends along the way, that might level the playing field.