I have never raised any to breeding age but I think the age is more important than size. Many of my breeders were only 3 to 3 1/4 inches. I raised some to 3 inches in a year and a half and they were shaped much different than an old mature 3 incher. The young ones were flatter and the older breeders were humped. I don't believe that the young flat ones could support eggs -not enough room in there. I would guess five years of age would be minimum for a female to lay eggs.