What is the youngest rat you have seen with mammary tumors? I have two females that are both rescues. I have no clue how old they are. One I got from the spca and the other I got from a lady that found her in the parking lot near her work. If I have to guess how old they are I would say around 1 year. I've only had them for 3 months. They are both healthy other wise and very active. They have both already had one growth removed and I have just noticed they both have another growth. I haven't taken them to the vet because I just noticed them and it is also midnight. I am going to make an appointment with my vet first thing in the morning. I was just wondering what others have experienced in the past. These are my first girls.