Why is my baby aquatic turtle biting others? How can I stop it?

Separate out the aggressive one. It could be nipping due to hunger, stress or aggression. Consider hand feeding to avoid accidental bites as well as dividing them into two tanks to provide more space. As they mature, they will require a significantly larger space. This number of turtles ideally will need a tank in the hundreds of gallons range or a small pond. A general guideline is 10 gallons per inch of turtle.
Sexing turtles is easier when they are mature. Without knowing the species, it can be difficult, but in general, females are larger than males. Males have a shorter, thicker tail in comparison and usually have longer front claws. A female will have a flat plastron whereas the male will have a slight concavity. Males have a v-notch at the rear of the carapace to accommodate the tail when mating. Lastly, you can check the cloaca, or vent. Females will have a rounder vent closer to the body whilst males will have more of a slit farther down under the tail. All these differences are best seen when comparing two turtles or a proven pair.