User blog comment:Jimeee/Skyrim 1.9 beta update adds Legendary difficulty and raises level cap (Updated Apr 8th)/@comment-99.194.116.134-20130327204054/@comment-75.66.126.122-20130328045615

Followers were never meant to level up ( it was assumedly scrapped during developement), merely to match your level when first met.

If you- or anyone reading this- wants to change their level or fix the Marked for Death glitch and the multiple "hidden" inventory glitch, Wabbajack them into a bunny thus reseting them.