Yes, this is exactly how it will work. Unless there is some mechanic where if you clean out a dungeon that it respawns with completely new monsters and creates a new area level.
I am certain however, that if you enter a dungeon, leave without killing everything and then come back it will be at exactly the same level it was the first time you entered it. same applies to dragons as confirmed in a few different interviews.
Hopefully there will be some dungeons with very high starting points like a level 40-50 dungeon, but I suspect many people who play the game will never get anywhere near character level 50.
rather i believe it'll work more like this:
"from what we've been reading i predict that the loot system functions similarly as to how the leveling system is supposed to work.
in other words...
loot is leveled according to the location it's in and according to the player's own level.
ex: in a cave filled with enemies of let's say... level 3 to 6, the level of the loot found in this location would be between 3 and 6 so probably iron or leather equipment.
if your character is level 3 or lower the loot level you find in this location would be at its weakest and conversely if it's 6 or higher the loot level would be at its highest but never stepping out of the bounds imposed by the location level.
special loot like unique weapons and armor are an exception to this rule making their stats fixed and not dependent on the leveling system. "
going by this logic, if a location has a lv30 reward it's easy to assume that said location would be filled to the brim with lv30 enemies. if a lv5 character enters that location, he'll most likely die in horrible horrible ways until that player either levels up his character to match the enemies in said location or the player is that good and can retrieve the hard earned reward by pitting his character against a mob of enemies that are definitely above his own level by a large margin and win.
levelling would also differ between creatures so that rats would only go from say level 1 to level 10 and in a level 30 dungeon you might encounter some rats but they would be wouldn't be the main enemy for that dungeon but rather a "flavor" grunt enemy and at most only at level 10.
again, this is not fact but it's what i can surmise from the info that's been released until now.