At URL
http://www.kernel.org/git/?o=age
one can see the various repositories, sorted by last-modified time.
However, AFAICS, only the 'master' branch is checked. This causes an incorrect age to be returned, for kernel hackers who always do work in topic branches ('master' is Linus vanilla kernel).
Thus, the age on my repositories is only updated when I update the 'master' branch with new linux-2.6.git goodies. Development work I do in topic branches is never noted in the 'age' column.
Jeff