Most cats have a built in instinct to hunt & can start at any age, some sooner than others depending on the availability of mice. My 2 yr old has been mousing since he was about 6 months old & is such an expert that my neighbours borrow him to hunt in their gardens ( field mice) & he often gets christmas presents from grateful owners. Personally I don't like to see it & rescue whenaver I can, subject to the condition of the poor creature in which case I kill them quickly rather than see the cat playing with them as they mostly kill for sport rather than for food. I have noticed too that crows & jackdaws etc soon carry them off....Spinner.