from this site http://www.seocompany.ca/pagerank/page-rank-update-list.html it show back to year 2000 but i'm not really sure though.
That source has to be smoken something, if thats what it says. I cannot remember perfectly, since I got into webmaster business within a year ago, but I'm pretty sure that it was first introduced around 2004, someone can correct me if I'm wrong.
I'm not sure but as far I know the Pagerank was first introducted in 1997. The search engine idea that Page and Larry presented was this Pagerank concept. Google search is based upon Pagerank so probably it came up when Google was launched.
http://en.wikipedia.org/wiki/Pagerank @anions It won't hurt your brain if you try to gain more knowledge. - DarkX
why are you guessing when other posters are looking up sources to get the answer? This is an interesting source on it too http://infolab.stanford.edu/~backrub/google.html