Will my two British Shorthair kitttens start mating?

I have a 5 month old BSHblue boy and a 4month old BSH lilac girl (different litters). im thinking in the future i may want to mate them to produce a beautiful litter off fine kittens...however im thinking they may start breeding to early...when could they start and is it safe! ideally i would wait until they are both a year old... but i wont be able to stop nature if they are both living together as indoor cats!