Google's interview process is a magical opaque box. You have no idea what happened. All kinds of people who seem like an excellent fit somehow end up getting rejected multiple times. And I mean excellent fit in the most cynical worse-case way possible. The "right gender", "right race", "right age", "right school". I interviewed at the big G back in 2012, did 2 phone screens, made it to the on-site, thought I did well... but no dice. It's like an f(x,y) then divide by z algorithm. X = You, Y=Them, but Z is generated by a random number depending on the zodiac sign of yourself and 2 random people somewhere on earth. You can do everything right, Z = 0 and segfault you're disqualified. Just gotta try again. Not excusing Google per se, but I doubt age-discrimination. They're just crazy like that.
Of course, I don't know all the facts and it would probably make a lot of people happy if Google made their interview process more understandable. Maybe G's HR should stop getting people's hopes up saying "“embarking on its largest recruiting / hiring campaign in its history,” and “you would be a great candidate to come work at Google.”
Just tell them the truth. "We're gonna interview you, then at the end you're gonna pull this slot machine and you better hope it comes up 7-7-7. Because that's basically what we do after you leave the building."