to get to level... I don't know... 11?
Regarding the tiers, defacto, they are:
- 1 to 4
- 5 to 8
- 9 to 12
- 13 to 16
- 17 to 20
These tiers are the rythm of the 5e gaming engine, the feats, the proficiency bonuses, and how the class features time arond them.
Because the frequency of games falls off around level 8, this "mid tier" of levels 9 to 12 is significant in 5e concerning who is playing it.
Meanwhile, in earlier D&D editions, 5 to 8 roughly corresponds to the "sweet spot", and 9 to 12 to published "high level" adventures.
Officially, 2014 has the tiers be 5 to 10 and 11 to 16. Recently, there was talk of 1 to 5, 6 to 10, 11 to 15, and 16 to 20.
But defacto, the rythm of mechanics across all 5e classes, and the fact the mid tier of 9 to 12 feels so different from 5 to 8 before it and 13 to 16 after it, the tiers split up most usefully by 4-level tiers each.
Each of these five tiers, including the mid tier, contitutes the mechanics for a completely different genre of fantasy.