Should I retire followers after I've leveled up significantly since their joining?

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) enable disable into the console.

Yes, that's mostly correct. However, my understanding is that it's your level at the time they're first loaded (i.e. when you visit the area they're in) that controls their level, not your level when you hire/enlist them.