Cohorts are loyal (almost fanatically) adventurers.
Followers are the folks who run your castle while you are away, and things like that. Followers who are allowed to come along on adventures and actually help in any way other than holding the horses and the like, die.
Think about it. 5 1st level followers go adventuring with their 10th level leader. Can you say "slaughter?"
Simple problem, simple solution.
Also, keep in mind that followers, unlike cohorts. may come and go based upon your current leadership score and, by inference, will leave if they are too high a level to be your follower any more.
If it helps, a definition of Cohort is:
Cohort: a supporter, accomplice, or associate of a leader, especially one to whom special treatment and preference is given.