MrZoink
First Post
I think it just said to treat the cohort as if it were 5 levels higher for the purpose of the initial recruiting (e.g., the max level cohort you may recruit). The justification was that a character that belongs to a PrC is more of an individual, less likely to be a sidekick/follower.
Just to clarify, since I looked it up, the rule I believe you're referring to is an option about having followers with PrCs. Nowhere in the ELH or DMG does it refer to cohorts having a penalty for PrC.
Just wanted to point that out since there is a big difference between cohorts and followers.