Some platy fry proliferates and may show signs as soon as three weeks after hatching. Other males may not begin to show symptoms until six weeks old. In addition, men are much slower than women when getting things done. They may not grow as big or quickly as female Platy fish, and it will likely take them longer to reach a mature color like that of a female Platy fish. Some experts think that this process could take up to three months to finish. A lot of the time, when male platies are born, they don't start to look like males until they are about three weeks old.