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It's an interesting exploration into what Google doesn't index -- and as one of the experiments, it uses the word Googlology, coined right here on WebmasterWorld.
According to the Google front page, Google indexes 3,083,324,652 web pages out of an estimated 10 billion or so pages that are actually on the Internet. We ask the question, "What does Google leave out?"Article at Microdoc News [microdocs-news.info]
That article would suggest that you need recent links or updates to your older pages.
I wonder if their WebWolf crawler obeyed robots text?
I was also interested in the invisible web according to Google a short while ago: [webmasterworld.com...]