I've read the instructions on installing older versions but it appears that 2.4 is no longer available in the repos?
aptitude -y install mongodb-org-server=2.4 Unable to find a version "2.4" for the package "mongodb-org-server" Unable to find a version "2.4" for the package "mongodb-org-server" The following NEW packages will be installed: mongodb-org-server 0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded. Need to get 0 B/9,416 kB of archives. After unpacking 23.7 MB will be used. Selecting previously unselected package mongodb-org-server. (Reading database ... 33931 files and directories currently installed.) Preparing to unpack .../mongodb-org-server_2.6.3_amd64.deb ... Unpacking mongodb-org-server (2.6.3) ... Processing triggers for man-db (188.8.131.52-1) ... Processing triggers for ureadahead (0.100.0-16) ... Setting up mongodb-org-server (2.6.3) ... mongod start/running, process 30690
UPDATE: I've also filed a ticket with Mongo's JIRA, which has received a reply.