Many go by weight instead of age. Look up the senior weight for your breed and when they reach that they are ready to breed. Generally though at the earliest it's 4-5months for smaller breeds and 5-8 months for larger breeds depending on just what breed we are talking about.
I breed mine, meat and pet sizes, around 5 months old. Some get pregnant right away and some don't catch for a month but all have had a litter before they're 8 months old.
It's generally the giant breeds like flemish and checkered that people wait 8months to breed. They take so long to get to senior weight and come in to condition. Most of your commercial type breeds are bred months before that.