Some brief research has lead me to the conclusion that, while the strengths of a follower match your current level at the time of their joining you on your quest, they do not level up through the rigors of questing as does your character. If this is true, it would appear to make sense to retire a follower after you've gone up X number of levels. For example, it wouldn't be practical to take Faendal from Riverwood with you from the start of your quest all the way up to your character's 50th level since your follower would be uselessly outmatched probably at around level 10 or 15.
While this is generally true, there is one item that will forcefully reset your followers (to reflect your new, higher level) - the humble Wabbajack!
To use, you want to wabbajack your follower (might want to save first, so you don't accidentally slay them) until they get hit by one of the transformation effects - when they turn back into their normal self, they'll have been updated in level, the same as if you had clicked on them and typed (in sequence)
disable into the console.