I'm not exactly scientific about it. I do prefer to let females get to 10-12 weeks before breeding BUT, if I need to start another litter and don't have a previously bred female or an old enough virgin, then I use whatever I have. I try to keep a tub with a dozen(ish) females in it to raise up for breeders. If I've been good about keeping it stocked, then there are older females ready to go. I select by physical size and color needed/wanted for the breeding tub she's going into.
I "cycle" breed, introduce a female to a male for impregnation and remove female for raising litter. Each female has a history tag that follows her arround so I know some details about her. Tag indicates color and duh, if I put two females the same color in the same tub, which is which? Sometimes can do with a older female and a much younger female
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Thanks,
Dave Colling

www.rainbows-r-us-reptiles.com
0.1 Wife (WC)
0.2 kids (CBB, selectively bred from good stock)
LOL, to many snakes to list, last count:
12.25 BRB
12.14 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 