Weight is key, but it needs to be a healthy weight. 1500g is usually considered a minimum weight and yes some females will reach it by 18 month or even earlier. If she has been power fed though and is a year old weighs 1500g and is FAT, she would probably not take anyway. If she is a year old and a svelt 1500g go for it. I have 3 yr olds that still may not make it though, they only weigh 800g picky miserable eaters

My 04 CH though that is a rat eater is a healthy 1090g already and will be paired up this fall, wether or not she gives me eggs will be another story, that is all up to her.