Ah, okay. Recommended breeding age is not the same as the age they're first able to breed. Males that are bred to young tend to be unsuccessful, and females that are bred too young might be stunted or have more problems with the litter. Another example here is human girls *could* get pregnant as soon as they start ovulating, so for some 12 years old. That doesn't mean that a 12 year old girl's body can handle a pregnancy well though, and many birth complications tend to happen in very young girls who are pregnant.
Hope that clears it up. My girl Mocha hit puberty at about 3 months and was spayed at 4.5 with, as the vet said, a fully formed and perfect uterus and ovaries. She's one of those early girls. That's why it's recommended to separate genders at 8 weeks, minimum by 12 weeks to prevent unintended pregnancies.