Based on arcane and divine spells/powers I guess.
I figure that's because the Balor is always supposed to be a top-level Demon in any version of D&D, it's level/CR/XP total is arbitrarily set towards the top of the power that PCs are ever expected to get.4e Epic is a rather odd mix of 3e "Very High Level" (16-20) and "Epic (21+). If you look at the 3e monsters translated into 4e, some high-teens-CR monsters got turned into High Epic (eg the Balor) where they get to rub ELs with Atropals and other 3e-Epic stuff.
I'd put 'em at 1-6, 7-13ish, and low teens+. 4th level spells are kind of a turning point, with things like Scrying and Solid Fog that fighters just can't do anything about (flying foe? arrows. Fireballs? reflex saves and hit points. Solid fog? Screwed.), as well as Lesser Globe of Invulnerability, which puts the nix on lower-level casters. Phantasmal Killer, for all that the double-save is terrible, introduces save-or-die, Enervation introduces negative levels, Dimension Door introduces teleportation, and Stoneskin and Polymorph start making melee arcanists a very real possibility. On the divine side, Reincarnate appears as the first return-from-death ability. 4th-level spells are the heralds of a new tier of caster power.
I would argue that the second shift occurs with 7th-level spells, mainly Limited Wish. That's the second shift for casters, where they go from well-defined spell effects to open-ended ones, and the gates of Crazytown are shattered.
I would say that Pathfinder paragon levels begin at level 9, as soon as the wizard can cast Teleport.
Epic levels start at level 17.