Google is an aberration. I work with many different companies, and the average age can vary greatly according to culture. Google has a very young average age, heck I think half the males there can't even shave yet. Startups also tend to be very young. But then go take a look at medical technology companies. A much higher average age. Animation studios: very young. Petroleum engineering: higher age. Financial trading: somewhere in between. Military contractors: much older. Other miscellaneous companies I've seen have also ranged from the very young to long in tooth.
I am talking about the SOFTWARE DEVELOPERS in these companies.
I think the two factors that push the average age downwards are: 1) The trendiness and hipness of the company. Kids want to go work for Apple and Google, and not for IBM or Oracle. Older workers shy away from these because they feel uncomfortable. Then there's 2) the cultures at software companies that emphasizes newer languages, technologies and platforms. "Newer" being relative of course.