I know this thread has been left for some time but I felt like adding some experiences and thoughts of my own to it, just as an extra to read for others.
I had a 2 year old (close to 300 grams) AND a 4 year old 550 gram virgin lay slugs this year... They both had been with the same non proven adult female temporarily (though not at the same time), so I had that one probed when the opportunity rose and she really is a female and the egg layers were indeed still virgins beyond doubt. The 2 year old did fine, only retained one small slug for a couple of days close to the vent but got it out herself. The 5 year old had made 20 (probably) slugs inside, laid only 10 herself, a couple more with my help but died from binding with 7 stuck ones eventually. The smaller girl was not very strong muscled but right sized, the 4 year old might have been a bit too thick around the girth. What does it say about the 3x3 advice? That it's not all there is indeed...
Further, I own a '05 female which had spontaneously laid slugs in '07 at her previous owner though she weighed only 198 grams in Dec '07 when I bought her being back in weight...*ouch* . So I bred her in '08 when she was a bit heavier (about 250 grams and almost 3 yrs old) to prevent another clutch of slugs, however she did lay slugs though I had observed successful mating (the male was an '06). In '09 I bred her to another 2 year old male and she laid a proper 9 egg clutch and 1 or 2 slugs. This year I bred her to the same, now 4 year old, male and she gave me 10 good eggs and 2 slugs.
So, why did the female produce slugs with this male in '08 but fertile eggs in '10? Why did she lay a clutch with slugs in her 2nd year of life ('07) if she apparently was not ready even in '08 when she laid slugs from a later proven fertile male? Of course, the male might have been too young, he did give slugs with another, 4 yr old female the following year ('09), but somehow in '09 and '10 they were both ready after all?
My point is: though the 3x3 might be a great guide line, please ask experienced breeders to advice you as long as you don't have a clue yet how to see if they are ready body wise and they are ovulating, preferably showing your animals to them. It's a matter of intuition, grown by experience and knowledge from experienced people. I have gone through a breeding cycle with my snakes 5 times now, probably adding up to about 25 clutches and only this year I finally got a grasp at what ovulating females look like and what they look when they are really close to laying. That helped me conclude that the virgins above were going to lay so I could provide them with laying boxes in time.