The key to fossilization lies in the concept "preservaton" - most organisms are not preserved at all - only those which end up in a sedementary environment where they are proteded enough from the forces and organims which would normally destroy them, become "fossils".  As to the 10,000 years old criterion, there is nothing magical or very signifigant about this figure on a planetary scale, as it doesn't constitute a geological transition exept for the very end of the most recent ice age in the northernmost portions of the world.  The point where a subfossil could be said to have crossed the line and become a "true" fossil, is rather subjective in practice - perhaps the 10,000 year mark is more one of convenience, and the fact that the geological time-charts, with their named periods, is a European invention, and the end of the most recent ice ages is more important in Europe than, say, China.
-Ross M.
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