Cat age and breeding saftey?

i have owned over fifty cats and raised/delivered over 8 litters.i know quite a bit about cats through personal experience and books.when i delivered every litter the female was already pregnet.i own two cats right now.one is pure white,short hair,deep blue eyes,light pink nose,pink paw pads,and seven toes.my other is medium hair,circular leopard like markings,magenta nose,light auburn eyes,and black paw pads.they both have beautiful features.i want to breed them because the kittens would be gourgous.i am familiar with the local shelter and give/adopt animals to them often.i would give kittens to close friends and the rest to the shelter.that way i know all of them will have good homes,medical care, and the shelter will make eighty off each kitten to go to the animals.the time span for adoption of young kittens is 2 weeks max.they get adopted like candy from a office candy bowel.i want to know what age can a female safely deliver kittens.PS i would like a sure answer not "i think a year old but im not sure"