I believe you will find verbage about historical data in Google's patent. http://www.seomoz.org/article/google-historical-data-patent
You can also read the following from Google's Page Rank Patent 6285999
4. The method of claim 1, wherein the assigning includes:
identifying a weighting factor for each of the linking documents, the weighting factor being dependent on the URL, host, domain, author, institution, or last update time of the one or more linking documents, and
adjusting the score of each of the one or more linking documents based on the identified weighting factor."
It is not specific on if newer links are better or older links more valueable. Since Google does pay a lot of attention to trends and hot topics I think there could be an arguement for newer links being better, but perphaps only on a trendy base. Meaning new links = quick boost, but old links = authority.
