It depends on whether it's the books or movies. Book Sadako died when she got trapped in the well at 19, movie Sadako survived and lived for most of it, making her middle aged. The DBD Ringu chapter is based on both so we don't know which is true for the Sadako in DBD.

The good Sadako grew into an adult and became an actress, while the evil twin was locked up by Ikuma and drugged to stunt her growth. Akiko Miyaji, the fianc of the journalist whom Sadako killed, leads an angry mob to kill the evil Sadako, only for the twins to merge into one and slaughter her tormentors.