Captive red-eared sliders generally reach sexual maturity within 2-4 years. Male and female red-eared sliders look a lot like each other even after sexual maturity, but they do have some essential differences that will clue you in on their sex.
Male red-eared sliders have long front claws and a longer tail. They also have a concave plastron.
Female red-eared sliders have short front claws, a short tail, and a flat plastron. They also tend to grow larger than males.
These rules also apply to yellow-bellied sliders (T. scripta scripta) and Cumberland sliders (T. scripta troosti).