I have an 18 month old black pastel female who is now weighing 1850 grams the girl is a beast and is being bred by my bumblebee. Now these type of girls are special, I've noticed that there are three types of ball python female feeding responses, great, average and poor the poor ones usually change and get better after laying their first clutch.
The poor feeders can take up to three years to get to about 1200 to 1400 grams the average feeders here get bred at 24 months 1600 grams.
Now I'm not gonna sit here and recommend breeding 18 month old females BUT. If you have a female who is 2000 grams I believe it safe to say that you will get a nice sized clutch if she is ready only she decides that.