Some NPCs have a level that is a multiple of the player. When the player levels up, how do those NPCs' stats get updated? Does the actor keep their old level until a cell reset, or is it immediate? Followers seem to level up immediately when the player does (at least, Serana does; who cares about other followers).

The reason I'm asking: I'd like to set some actor value to a function of their level, but I want to make sure that it gets updated when that actor's level changes. If the actor's level DOESN'T change, and a new, higher-level version of them gets spawned on a cell reset, then I don't need to worry about anything. But followers concern me.