Most commonly found livebearers can be sexed in the same way at just about any age. The picture above is of black mollies. The fin circled is called the anal fin. As you can see, on males it is rather pointed while on females it is a rounded fan shape.

Here's some more pictures for you to practice on.

Male Platy

Top: female platy Bottom: male platy
These two pictures fromColleti's ode to the platy

Male swordtail. Male swordtails have the long tail in addtion to the pointed anal fin. Females do not have swords.

Female Platy
The above two pictures and the black molly picture from the American Livebearer Association.

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