A person's reputation tends to rise together with the age. The older one is, the more opportunities one had to do notable things, and to meet people who could appreciate those things and tell others about them. So this makes sense.
An online document's reputation also tends to rise together with the age. The older the document, the more documents link to it, and the more documents in turn link to those documents, raising the old document's PageRank. So this makes sense, too.
The paradox is that the older documents are written by the younger people. That is, it is one's younger version that wrote one's older documents. So the documents with the most reputation will tend to be written by people (or more precisely snapshots of people) with the least reputation; one's dumb young stuff may well pop up first in a Google search.
(Not that there aren't any counter-tendencies to cancel this effect at times; my old anxious, moronic report of an imaginary bug in ALL CAPS no longer shows up in my egosearches. So no, I'm not bitter. In fact, Google loves me more than I deserve – for instance, my review of Extreme Programming Explained has appeared in search results right after the Amazon entry for the book ever since I published it, and I've only skimmed through the thing. The only thing that bothers me in the SEO department is that the search for "C++ FQA" gets corrected to "C++ FAQ" – didn't expect that once the query got past the spell check barrier. I hope my collegue's riskily named DreamPie project will not experience a similar setback.)