its more about weight to length ratio. with age its if the are sexually mature or not. the question is if she has enough weight to support her eggs. 1200 grams is fine as long as she not too long for her weight. you can still feed her if she will eat. don't pound her with large feeding at one time. smaller feedings more often is what you want to do. she can still gain a good bit of weight before shes ready to lay, depending where she's at in the breeding cycle. I had many females breed at 18 months around 1500 grams and had 7 to 8 good eggs no slugs. and they went on breeding the next year and did well also. 1200 grams expect less eggs.