Q u o t e:
Please correct this. It's 75 minimum, not 78 and I almost didn't try on my shaman because I thought I had to be 78.
The minimum level requirement for the final battles was originally set at level 78, though was moved to 75 when implemented. I just confirmed this with the events' designers.
It's a small mix-up, but the current minimum level requirement is indeed 75 for the final battles and level 1 for the majority of the lead in quests. My previous post has already been updated. :)
Edit: Updating level requirements.