I know someone who was hired at Google right about the time he hit 60. Really smart guy which is how he got in, I suppose. He left after a couple of years. He hated it. Said the ageism was rampant and they didn't even try to hide it.
I know another guy who is a top C++ developer (books, conference talks) who started at Google when he was around 45. Similar story. He hated it, left after a couple of years and is now Chief Scientist at a much smaller software company - a gig he seems to relish.
So you could try to get in in your 40s or 50s, but it's likely that the culture will not agree with you.
Not that I would have qualified, but years and years ago when I looked at a Google recruiting page, it featured a picture of "field hockey in the parking lot at lunchtime", IIRC. I took that as a hint about age preferences.
I know another guy who is a top C++ developer (books, conference talks) who started at Google when he was around 45. Similar story. He hated it, left after a couple of years and is now Chief Scientist at a much smaller software company - a gig he seems to relish.
So you could try to get in in your 40s or 50s, but it's likely that the culture will not agree with you.