Some people only figure out that being a founder is a possible life-path when they're already in their 40s/50s. You can't plan your life around things you don't yet know you'll want to do.
Presuming that every founder is someone who has wanted to be a founder since they were in their 20s--but for some reason just didn't perhaps get the chance until their 40s/50s--is exactly ageism.
No. If people were being actively excluded purely on the basis of age, that would be ageism. The fact that some people plan better than others is just life.
What is "age," practically, other than the accretion of responsibilities?
I don't think any of the people who worry about being impacted by ageism are speaking about age per se. They're more concerned with... let's call it "overtime-ism": a strong bias in favor of people who will be willing to work overtime. The video-game industry, Japanese megacorporation culture, and a large segment of silicon valley are all afflicted with overtime-ists, which seems to have a good overlap with where allegations of "ageism" are aimed.
Presuming that every founder is someone who has wanted to be a founder since they were in their 20s--but for some reason just didn't perhaps get the chance until their 40s/50s--is exactly ageism.