Young preys bones have not hardened yet. It doesnt mean they have less calcium ... it means that they are easier to digest and break down. Less calorie expenditure on digesting prey means more calories used for growing.
Depending on the age of the mouse it could even have less calcium content than an equal sized rat. Old (geriatric) prey usually have brittle bones (like in elderly people) due to calcium loss over thier life.
In general younger prey has more vitamins and minerals, better nutrition, and are easier to digest. Primarily because they havent used their stored reserves yet.
Bryan