Hah, a HP discussion.

Let me toss in my 2 copper pieces.
While neither Albus nor Voldemort seem to be epic level, I think 5th-6th is too low for them.
Disclaimer: I may get a bunch of levels requirements a little off the mark, since it's been awhile since I played 3.5
The main problem with trying to convert anything to a given RPG system - or at least one that uses powers tied to level-based progression - is that if a given power appears analogous to power X in the RPG system you're using, and you can only use power X at level X, then that character must be at least level X, even if nothing else about them is characteristic of the powers and abilities of level X.
*Voldemort can use large amounts of Fiendfyre, which could be an equivalent of hellfire, and iirc, small explosions of which are level 4 spells.
Voldemort never used fiendfyre (at least, not in the books). The only person we see using it is Crabbe, Malfoy's toady...which is a case-in-point for the above problem. Since fiendfyre is basically hellfire, which is a 4th-level spell (
Fiendish Codex II, p. 101), then is Crabbe at least 7th level?
I'd say certainly not, considering that he doesn't seem to have the requisite intelligence, or show any other particular aptitude regarding spellcasting.
*There have been claims of Voldemort's horcruxes being similar to a lich phylactery. It's not a perfect analog, but that puts him to at least 11th level.
Now this is a good argument for Voldemort being of a higher level. In practice, a horcrux does seem similar to a lich's phylactery - it's a magic item that restores the creator should he be killed. So then he must be at least 11th level, the minimum level necessary to make a phylactery (and be a lich, though he apparently made one without becoming undead)...or that's the idea, at least.
Personally, I think even this is open to a fairly wide possibility of interpretations. For example, it's easy to say "but wait, Voldemort had SIX horcruxes (seven, if you count Harry), so that's like having six phylacteries! And the only way for a lich to have multiple phylacteries is to cast
aumvor's fragmented phylactery (
Champions of Ruin, p. 37), which is an epic spell, so he MUST be an epic-level character!"
From there it's a short trip to saying that everyone else must have a comparable level to be able to stand up to epic-Voldemort at all.
*Dumbledore's trickier. Voldemort can't beat him in a frontal fight, but that's probably because of the Elder Wand (made by Death most likely implies artifact-level item). Even with the wand, though, Dumbledore can't seem to beat Voldemort. He does seem to outthink Voldemort, but then that's probably just because of a higher int score.
All in all, my guess is that Voldemort's at a higher level (more people killed=more xp? XD). Voldemort at level 13, and Dumbledore at level 9-11 would be my guesstimate.
I tend to assume that unless there are multiple instances of characters using a variety of high-powered effects/abilities, that they're of a fairly low level. How do we account for low-level characters using occasional high-level effects? I chalk it up to customized differences in things like feats, magic items, etc. and the occasional natural 20.
How was Crabbe able to use fiendfyre? Well, fiendfyre isn't really that high-level of a spell; it's a lower-level version of
hellfire (hell-burning hands?) that, according to the book, got out of control when it set the surroundings on fire and spread, with the spreading fire retaining the hellfire's quality of not being counted as fire damage.
How was Voldemort able to have multiple phylacteries? Well, compare the effects of them to a lich's phylactery. They didn't restore Voldemort to his previous state, good as new, 1d10 days later...he was stuck as a creature that was, in his own words, less than a ghost. So then these weren't full-power phylacteries, and he wasn't at least 11th level.
The Elder Wand? It's powerful, but to the tune of maybe adding a +2 to spell DC's cast through it, or maybe treating spells cast through it as +1 or +2 caster level higher than normal.
It's things like that why I don't see any of the characters in the Potterverse having levels that are anywhere close to the double-digits, let alone epic.