How old will Platy fry have to be before you can see if they are males or females?

Are the fry 'fully formed' when they are born, or do they develop the visible differences as they grow? They're about 1/2 inch long now, and getting nicely colored.I need to see for sure which are which, because my friend wants only the males in his tank, and he's waiting for them. Meanwhile I was wondering, if they are born male/female formed, or if it develops later, like going thru puberty?

keep an eye on them. you should be able to tell in about a month. you will then have about one more month to get them separated, or you will have more fry on the way.good luck

it depends on what kind of platy fry they are. if they are not orange or red or any other color bright yet then you should know by the small grivid spot above there anal fin. also all the babys will have a fanned out tail. once you start to see the tails stiffin back adn not become fanned out constantly then you know they are males. good luck with your fry hunn. if you ever have any other questions about your fry or any of your fish let me [email protected]~Ashley