How much time does it takes for a gold fish to become an adult?
This question is actually quite an important one.
Many Goldfish publications quote the age goldfish can be bred is between 1 and 2 years of age.
Looking at my 6 month old Black Moor juveniles, some of the males are starting to show breeding tubercles on their gill plates and the leading edges of their pectoral fins. This would suggest they would be able to be bred next season, so they would be adults at somewhere between 6 months and 1 year old.
These Black Moor have been kept in optimal conditions since hatching and fed primarily live food.
This is the reason the males are starting to show their tubercles this early.
I have used 1 year old males for breeding quite successfully. As for females, I find two years old is better as younger females seem to lay quite small numbers of small eggs. The fertility rate is also low.
If Goldfish aren't kept in optimal conditions, they will still become adults at between 1 to 2 years of age, but they won't get into breeding condition in spring.
Some goldfish kept in bowls or aquariums and only fed prepared foods never show adult characteristics.