During this early stage of adolescence there is a sharp increase in physical growth. Childhood spurts start at around age 10 for girls and age 10 ½ years for boys. Changes occur in early adolescence at around age 11. Girls become taller and heavier. Their pubertal growth spurt occurs about two years earlier than that of boys. At around age 14, however, the boys' growth spurt begins again and they overtake the girls, as the girls are almost finished growing. Body changes occur during puberty that reverse some of the basic growth trends of childhood. For example, the hands, legs, and feet accelerate and then the torso, resulting in the adolescent height gain (Wheeler, 1991). This explains why adolescents often seem out of proportion for a while. They are awkward and gangly with long legs, and feet and hands that don't seem to fit the rest of their body. Then the body fills out. This explains the need for clothes that change sizes with the filling out of the frame (Berk, 2000). Boys tend to end up larger than girls. This is a result of two extra years of preadolescent growth. Their legs are longer in relation to their body and they have broader shoulders in relation to their hips. Meanwhile, girls end up with hips that are broader in relation to their shoulders and waist. This is a result of the sex hormones acting on skeletal growth. Gains in large muscle development have leveled off by age 14 for girls. But, boys continue to experience a dramatic spurt in strength, speed, and endurance.
It should now be much easier to understand how all these changes will obviously affect your posture.