I have a few websites aged from 6 months to over 3 years old. From what I have noticed, site age is a significant factor with Google for the first few years. After around 3 years I think things become virtually a level playing field e.g. a 3 year old site can perform well for a particular search term against a site which is much older. I would guess that Google is applying some sort of logarithmic filter against the age of the website, and as the website matures to around 3-4 years the impact of the filtering is minimal. The age impact seems much less with other search engines, particularly Yahoo.