In chicken, both the hen and rooster when reach age around 6 months old, they can start to breed. Young rooster might needs few more month of age to produce fertile eggs. I wish this is the case for the white-rumped shama, but it's not! :)
As few others who breed this bird before me, and from what I learn, the female can start to lay egg at around 1 years of age (the following season after the first molt), while the male needs to be around 2 years of age to produce fertile eggs. In this breeding season, I have one pair that the male is two years old and the female is one year old. Both produced babies in their first ever clutch which confirm with what I learn. In other pair, both the male and the female are one year of age and in their first clutch, as of 16th days of incubation, no eggs hatched!
In conclusion, young shama can start to breed successful at an age of one year old in case of the female, and at an age of two years old for the male. As with other birds and other animals, first few years of breeding might not yield the most offspring. Too old or too young of a bird won't give max potential to produce offspring. I suspect that the shama will reach their full potential after 3-4 years of age.