Currently, characters at levels 70 and 80 cannot join the arena. The Blizzard UI indicates that the arena feature becomes available at level 70. However, attempting to queue results in the message “failed to join instance group.” Since this feature was available in the original Cataclysm and there has been no announcement about its disabling, it is reasonable to conclude that this is a bug.
While some may argue that only a minority of players enjoy twinking and non-max level arenas, it’s important to remember that these players are also subscribers and pay the same subscription fee as everyone else. Since this feature was available in the original Cataclysm, Blizzard has set the expectation that it would be available in Cataclysm Classic as well.
It would be helpful if a developer could confirm whether this is indeed a bug and work on a fix or clarify if this is working as intended.