I found a nice command from an answer on StackOverflow that allows you to sort branches by date. I modified it slightly to also show the date when printing the branch information:
$ git for-each-ref --sort=-committerdate refs/heads/ --format='%(committerdate) %09 %(refname:short)' Mon Jan 14 23:46:15 2013 +0000 keyfile-seek-rebase Sat Jan 12 02:09:04 2013 +0000 perfdiag-mbit-fix Fri Jan 11 17:38:13 2013 -0800 keyfile-seek Thu Jan 10 01:05:43 2013 +0000 master
You can also set the date format to be relative (or other possibilities, see man git-for-each-ref):
$ git for-each-ref --sort=-committerdate refs/heads/ --format='%(committerdate:relative) %09 %(refname:short)' 3 days ago keyfile-seek-rebase 6 days ago perfdiag-mbit-fix 6 days ago keyfile-seek 8 days ago master
I added it to my .gitconfig file as an alias:
[alias] branchdates = for-each-ref --sort=-committerdate refs/heads/ --format='%(committerdate:relative) %09 %(refname:short)'
That allows me to just type git branchdates and get a nice listing of my local branches by date.