How old does a guinea pig have to be to breed?
You are asking the question the wrong way round. Female guinea pigs should not be older than 9 months as their pubic bones fuse making it difficult for them to give birth. Both mother and babies could die during the birth. Please don’t breed your guinea pigs as there are so many animals in shelters and rescues abandoned by their owners waiting for a loving home, if you breed more you will deprive them of a second chance for a happy life.