Be interesting to see what other say - since I am not a git expert - but you do have these options.
First find a commit which is two weeks old
git log -n 1 --format="%h %aD" --until="@{2 weeks ago}"
b4f2ff3 Fri, 28 Oct 2011 08:36:56 -0600
Show unmerged branches since that commit
git branch --no-merged b4f2ff3
  foo
* master
  views_to_tables
Show merged branches since that commit
git branch --merged b4f2ff3
  Hday.mt
  commod_stat
  commodity_stat_SP
  merged
  printscheman
  program_options
  release
  test
Delete local branch
git branch -d <branchname>
Delete remote branch
git push origin :<branchname>