I use a different kind of illustration for older years book. A good example of this is the Death Game book.
This 'choose your own adventure book' incorporates a darker, edgier illustration which is more realistic and scarier in tone. A lot of time is spent creating the world which these horrors inhabit, which give it a deeper feel.
This style is perfect for a young adult fictional story of if you are creating a game that needs a set of fantasy playing cards.