If you truely want to start a good honest kennel, wait until your pups are
at least 2 years of age. That way they are fully grown so when you do your health testing (hip, NCL, etc) they will be accurate since the dogs are finished growing.
Since you love your dogs so much and obviously love the pit bull breed, I'm assuming that you want to better the breed and bring more worthy pit bulls into this world. In order to do this, on top of health testing, you need to prove your dogs worthy. They may be worthy in your eyes because you love them, but you need to work them to make sure there are no underlying faults that you just can't see. By working them and showing them, all their traits and faults will be brought out and then you will be able to determine whether your 2 dogs are worthy enough to be breed.
I would suggest going to some kennels, talking to well known and infomed breeders and learning more. Go to clubs and shows and see what is out there and gain as much information as you can about starting a kennel, raising pups and breedings dogs.
This will take a few years, but if your dogs are awesome enough and worthy enough to be breed then it doesnt matter how long you have to wait cause the pups will turn out good whether your dogs are 2 yrs old or 5 years old
check out this link too for some more information
http://www.bulldogbreeds.com/discuss/viewtopic.php?t=2495