The story pivots on the specific age of the protagonist. Being 18 is a liminal space—you are no longer a child protected by innocence, nor are you an adult armed with experience. In the lore of the film, this makes the "mortal" ripe for the picking by demonic entities.
(Is it a survival horror, a puzzle game, or a text-based RPG?) demonic exam mayas shrunken mortal 18 better
For a mortal, being shrunken represents the ultimate loss of agency. You are forced to rely on wit and stealth rather than brute force. The story pivots on the specific age of the protagonist