Tweens are 8- to 12-year-olds. They are in a unique stage of development—more mature and capable than children but not as independent as teenagers. A key part of the tween years is puberty when girls may develop breasts and start to get their periods, and boys' sexual organs can change and their voices deepen. Amid these rapid physical developments, tweens go through an important period of cognitive development, too, gradually becoming more socially aware and capable of reason.