I agree with this. In FANNG old and young people don't usually compete. No one will interview someone young to a senior position and vice versa. It's a bit like in the army... There are way more junior positions than senior which skewed these companies towards the young. Also, for senior positions the interviewers must be senior too, which means they are not young either.

In other companies that's not always the norm. I worked in companies with senior folks in their late 20 / early 30s while a few 50+ developers were software engineers (not senior).