WordPress 2.5.1 was released on April 25, 2008.
That was 5835 days ago.

WordPress 2.5 featured “few-click plugin upgrades.”
“Your mileage may vary, plugins in mirror may be larger than they appear.”

For background, you may want to check out this post.

@nacin and @koop think you should update, but they might be a bit biased.