weight is more important than age. 2 years, and in excess of 1200 grams. But to play it safe, 1500 grams seems to be the popular consensus. I have a buddy (won't tell you who) who breed a few 1000 gram girls this year and got good results.. I don't recommend it though. Too young, and too small you run a high risk of complications. Especially for novice breeders.. Having stuck eggs can be a nightmare. You run the risk of losing both the eggs, and the mother.
The bigger the better IMO. My girls that are being bred this year are 5 years old and virgins.