Totally depends on the breed! I breed Ori's/Siams, breeds that do mature sexually very early on. My own girls call every 3 weeks approx. for 5-8 days each call. The youngest I have ever mated is 8 months and that was our Cointreau. She came into call at 4 1/2-5 months and then called religiously every few weeks. At 7 1/2 months I contacted the GCCF to get their advice, I also took her into our own vet to have her health checked, to discuss the calling situation and my vet advised me to mate her. GCCF said to go with what our vet advised! She was a good, strong healthy girl. Mated at 8 months, produced a wonderful litter of 4 babies who are now fully grown adults in their own homes. I make no apologies for mating a girl that young, it was the best thing to do for her health. I have also bred a Siam kitten who had her first full call at 3 1/2 months and needed to be spayed shortly afterwards. I can appreciate it is not the same for other breeds, but for my breeds, they mature early sexually. As a rule, with MY chosen breeds, I choose to mate them at approximately 11 months of age, by which time, most of them have called more than 4 times if not more. I've only ever mated a queen early once and that was the abovementioned case and I certainly don't regret it
I do hope I don't have to do it again, we were just bloody lucky she was fit, healthy, and mature enough to cope with a litter and she was (and still is) a wonderful mummy