From my research:
It appears that size is more of an issue than age. Most people that are having success with breeding have females that are over 6" and males over 5".
I have had success with males as small as 4.5 inches. My females began laying when they were almost 6 inches straight line measured and over 850 grams in weight. When I first received my females they were 4-5 inches long and around 400-600 grams. It took them 3-4 years to attain breeding size from here. They might have grown faster but I choose to brumate them every year for 3-4 months, so this slowed their grow rate a bit.
Size is more important for females than males. My male is about 4.5-5" long and he is a successful breeder The females that I am breeding are two that are around 7" and close to 2 pounds, and one that is over 8" and close to 3 pounds (the male is just over 1 pound).
I have heard that 6" is the the starting point for females, so your new little girl might need to grow a bit more before she will breed successfully, but you never know. Plus, you'd be surprised at the growth potential of some of those imports once they have stable, good food and a good environment.
I agree with Ryan, females have to be at least 6" however small males can still be sexually mature, but I think they have to be at least 4.5" (from my observation)
It depends on the subspecies as to when females are large enough to lay eggs. With T.h.horsfieldii and T.h.rustamovi it's 6" SCL with T.h.kazachstanica it's 5.5" SCL.