All the young are silver until they mature, at which time the males start to show some colour.
Under ideal conditions they are sexually mature at 3 months old and you can identify males and females then. However, if they aren't grown under ideal conditions, it can take 6-8 months or more before they mature sexually and show colour differences.
Keep the water warm (28C).
Do big regular water changes.
Feed them good quality food at least 3 times a day and they should grow pretty quickly.