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>