OT: IMO "Low Level" isn't really a meaningful concept in 4e; as opposed to "Low Heroic" or "Low Paragon". There isn't really a direct comparison with prior editions.
In 1e AD&D you were clearly High Level at Name Level, 9th, so Low was roughly 1-4 'pre-fireball', Mid was the 5-8 'sweet spot'. Although in the UK White Dwarf mag etc normally treated Low as 1-3 - 'Basic D&D' level, Mid as 4-6 - 'Isle of Dread' level, then High as 7-9, with 10+ Endgame, and that was more how I saw it.
3e retained 1e etc's spell power levels, so despite claiming that High only started at 11+ it really worked the same as 1e-2e; 9th certainly feels High in 3e, at least if you're a spellcaster.
If you look at who you typically fight in 4e compared to prior editions, low level foes like goblins, orcs, and gnolls go through roughly 1-7, by 8th you are routinely fighting mid level creatures like ogres & trolls, and high level creatures like giants come in as routine threats around 14th. Epic is its own ballgame, corresponding roughly to 3e official Very High (16-20) and 3e Epic (21+).
So, if you still want to use Low/Mid/High dissociated from Tier, I'd suggest:
Low: 1-7
Mid: 8-13
High: 14-20
Very High/Epic: 21+
I think what this demonstrates though is that pre-4e 'Mid' level clearly straddles the start of Paragon Tier in 4e, making it less useful as a concept.
Edit: Personally though, if I hear "Low Level" without context I read it as "Low Heroic" or around 1-4, effectively the same levels as in 1e-3e. Certainly talk of 12th level as Low gives me a slight headache from the cognitive dissonance as I try to imagine Calastyx as a 'low level' threat(?!?!).
IME at the London D&D Meetup if people discuss 'High Level' it means Paragon Tier, eg:
"I'd like to run a Paragon Tier game, but it's hard to start players off at high level in 4e".