IF you plan on separating by sex and do not want them to get pregnant, you need to start when they are three months old. Then keep a close eye on them until they reach around 6 months old. The warmer the water the quicker they mature and start to develop into males. Some will develop slower though and it can take up to 6 months for them to become identifiable as male. I have had molly fry look female one day and then the next they suddenly develop a gonopodium. I have begun to recognize the signs they are male though. It seems even before they get their gonopodium they start to chase the girls and fight among themselves over the females. I seperat those that seem to be showing that behavior even if the do not have the sex organ and they always develop into males.