I like thinking about the hierarchy and logistics of the Underworld and Reapers.
How they are given their targets, what forms they could take, if they just get them randomly, are assigned them at the human's birth, or if they compete with each other (and sometimes poach another's assignment.) Mori as First Apprentice is more a rank of her character & skill rather than age of existence. She has a vision for things that Death himself trusts, and he wants her to carry out those changes when they are both ready for her to take the throne.