If s/he is captive born, could be as little as a year. If wild caught, probably at least two years old.
It depends on how small a pinky you can find- if you have a local supplier who sells extra small pinkies, she may be able to take them now.
Look at her midsection- you want mice about that big around, or a bit smaller.

Cutting mice is yucky, but not nearly so bad ifyou cut them while still frozen.
I have a knife and cutting board set I use only for snake food, and even tho its just a paring knife, pinky mice and even rats cut into quarters lengthwise pretty easily when just out of the freezer. Let them defrost in their serving dish.

My Checkered was less than 8 inches long when he started taking pinky mouse quarters. He only got 1/4 to 1/2 pinky at a feeding.