Why are you not supposed to be there until 20? Crucible is balanced.. it doesn't matter so much when it comes to weapons and damage.. so a player level 6 can dominate players level 27. Doesn't matter.
Not sure why think you shouldn't be in crucible until level 20? I know people who have done Crucible almost exclusively.. haven't done missions really beyond the initial few.. then went on to Crucible and now they are level 25-26. This is why Crucible has it's own armory and quartermaster.
The crucible is NOT balanced between a low level an a lvl 20, much less a lvl 24+
you can get some kills but the high level has a huge advantage, if you don't know this, you don't know how the crucible "balancing" works and I suggest you read up on it.