As I never played Vanilla, and started on BC, I figured I'd do a dry run to get familiar with my class.
At level 31 different resources on the Internet say different things, and after dying many times trying to get to the DME gate was able to confirm that entry into Dire Maul East is gated at level 45. This means that in theory at that level I can get a group together to kill the demon Pusilin, which you have to chase through the instance, to obtain the [Crescent Key]. By looting that I can have access to Dire Maul West and North, and even do a tribute run at level 45.
This also seems to confirm that Stratholme and Scholomance will be gated at level 45, according to this one source which got it right on the Dire Maul minimum level requirement.
I'm only now level 33, so I think I'll get a group together for the Library Wing of Scarlet Monastery to obtain the [Scarlet Key] which will be needed for the Stratholme Live section.