It really depends on the species.
Motoros (most common) should generally be 12" in disc diameter before they are ready to breed. However, there is some variability there.
They have a tendency to mature some time after they are two years old. However, I grew out a pup (born in my pond) to 12" in one year and I have heard of captive bred pups giving birth to their own litters within 18 months of being born.
Feed them like crazy and do tons of water changes and it can happen faster.
Colin