They don't have to be any certain age. If she is big enough she will lay when she is ready. One of my females laid at 9 mo., 10 mo., 11 mo., and again yesterday at 12 mo. I finally bred her this last time so this clutch are all fertile. So what I am trying to say is yes, everyone recommends 18 mo and being fully mature but all girls are different. If they are a good size and laying on their own, there is nothing you can do about it.