I have three mice, all female and all over two years old.
I take good care of them and until recently they've all seemed very happy and healthy.
A while ago, two of them (Tibby and Pepe) got pretty fat over a couple of months but I just assumed that they were being lazy and eating too much. It wasn't until two days ago that I realised that there may have been more to it.

A couple of days ago I was looking in their cage and noticed that there was a bit of blood on one of the tissues in their bed. I assumed one of them had hurt their paw or something so was keeping an eye out for it. I then noticed that one of my mice (Pepe) had a small wound on her side and her body was looking a little funny. I took her out of the cage to examine the wound and realised that there was a much bigger problem! On her side, from under her arm, nearly to her back leg was a huge growth. It's like two big lumps, both about the size of her head. They're very hard to the touch but besides being annoying and uncomfortable don't seem to be causing her any pain. All the hair on and around the lumps is still there and there's no signs of balding or anything else. She's also gotten really skinny, when I hold her all the weight comes from the growths, not her actual body.
I initially thought it could be an abscess due to the cut but after watching her I think she created the wound herself from scratching at the lump. Sometimes when she's on the wheel or walking around her back foot gets caught on the lump and she scratches at it.

I read online that studies have shown that green tea can help to prevent cancer in mice and also reduce the size of already existing tumours (as well as strengthening their muscles which is a good thing also). So I bought some green tea powder and mixed it in with their food. They're still eating happily so they mustn't mind the taste. I've also been putting antiseptic cream on Pepe's wound in the hopes of helping it to heal.

Kudos to you for them getting to be over two years old.

I adopted three mice and they all suffered from mammary tumors. These are passed down from their mom and there is no cure. I took them to the vets to have the tumors removed when they got too large. The mice did well after surgery and, for one of them, I had tumors removed three times. But after each removal, the tumors returned even quicker, and quicker than before.

Have never heard about green tea preventing cancer. If your mouse has mammary tumors, green tea won't help. Polysporin (not neosporin) can be used to help heal the wound.

Best of luck.