It's a synonym for fmap. Lens' is a type synonym. age really has function  type forall Functor f. (Int -> f Int)  -> Person -> f Person. Applying k x gives me f Int and fmapping the constructor gives me f Person. If you want to read an excellent but challenging article on why lenses work this way click here. Otherwise the signature should give you enough to be able to play type tetris to figure out where the fmap comes in (hopefully). This isn't super beginner friendly, but it's where haskell seems to be headed these days.

Ben