"How old do they have to be before I try to breed them?"
If a female ovulates she is ready to breed, The same rules that apply to humans do not apply to snakesv in regards to developement and size. i have bred my Florida king females at 20" and got 6 and 7 good LARGE eggs out of them. These females all thrived and led perfectly healthy lives for many breeding seasons to follow. I will breed one year old females if I can get them to eat enough during that first year. This usually requires feeding through the winter and brumating for a short 2 week period in march.
With males it is easier because male brooks kings grow faster and large than their female counterparts.