Hi Ginger,
The reason for the discription is that the size of the food for snakes has alot to do with the size of the snake not the age. So by knowing the lenght and the size of the thickest part of the snake then we can give you the best answere.
The general rule is that you should feed the snake food (mice) that are about 1 1/2 times the size of the thickest part of the snake. About once a week.
I hope that this helps,
Jimmy C.