A 9 month old eating hopper mice is underfeeding it. My gal was eating hopper mice at like 3 months old. At her weight, she can eat weaned rats or small rats, depending how the person you get them from sizes them. At 9 months my girl was eating small rats. Now she is eating large smalls to medium rats once every 2 weeks except atm she is on a hunger strike and hasn't eaten for about 2 months yet she left me a big turd right in the front of the cage a couple days ago. Definitely up the size and my advice is try and switch to rats asap as it will make life cheaper and easier for you in the long run.