A friend in Silicon Valley told me that rumors are rampant that Google won't hire anyone over 40. That's just fear talking in a place where lots of companies are laying off people -- if you can pass Google's famous tests, why would it care how old you are?
That said, I have heard Mike Moritz, one of Google's VCs, say that he doesn't like to fund ideas ginned up by people over 25. And the Valley does have a kind of "never fund anyone over 30" credo. It likes young entrepreneurs, and at a certain point brings in adult supervision (like Eric Schmidt at Google).
So the Valley would have passed on Ford Motor, founded by Henry Ford a month before his 40th birthday. Sure, Ford has trouble now, but let's see what Google looks like when it's 105. Then again, Ford doesn't fit my over-40 criteria (and he started his first car company at 36).
After poking around for a day or so, I remembered Bill Gore, who started Gore-Tex maker W.L. Gore when he was 45.
Give me some more examples of entrepreneurs who started after 40 and succeeded. Let's inspire the 40-somethings in Silicon Valley and everywhere else (including yours truly)!