well it's not like they didn't know her age either, they saw that before they called her too
It's quite one thing to know an age beforehand, and another to experience the age firsthand.
Although how would they know the age beforehand? It's not legal to ask and most people don't say.
Even if explicit years aren't listed, it's usually not hard to decern how many decades of experience a person shows on their resume.
I think it can easily be that in-person, the group of younger people simply does not feel as comfortable with them. It's not even really age discrimination so much as cultural discrimination because the difference in cultural experience is so large... Frankly I don't even have an issue with it, because if a group is not comfortable working with you you are not going to be happy working with them either.
I totally agree that a lack of "fit" is almost always bidirectional. If they don't like you as an employee, you probably won't like them as an employer. However, this "culture" thing is totally bogus. It's a cop-out and a codewode for differences in gender, sexual orientation, age, race, family situation, appearance, height, weight, accents, religion, political views, hobbies, sports fandom, school attended, etc. It's another way to say that you are experienced and qualified in terms of skills, but I still don't want to hire you. In fact, someone of a different race would directly have differences in cultural experience, and that type of hiring consideration is illegal. There has to be a better codeword