I wrote:
Average was easiest, so I did that as a first try. It shows an initial upward trend, stabilising in early 2005 at about 300 days.
Here's a histogram series for the same data in the form of an animated GIF. It shows the distribution of editor age, averaged over a whole year, for years 2002-2005.
http://meta.wikimedia.org/wiki/Image:Days_since_first_edit_histogram.gif
-- Tim Starling