Jacob Marley
Adventurer
BTW, when I'm talking about assumptions, I'm talking about GAME ASSUMPTIONS, not what happens at your table. The game assumes everyone is the same level and has done so since 3e.
I can't speak definitively about 5e as I do not own the DMG, yet. I can, however, speak definitively about 3e and 4e as I have those books and have run those two systems. To state that 3e assumes every party member is the same level is incorrect. The 3e rules includes numerous ways baked in the system to cause a level disparity: Raise Dead, energy drain, and item crafting (to name a few) will all cause level disparity.
4e, on the other hand, does assume level similarity. It specifically says so on page 121 of the DMG under the heading: Absent Players (paragraph 2).