Final answer:
The abdomen normally moves with breathing until the age of 8 years.
Step-by-step explanation:
The abdomen normally moves with breathing as part of the respiratory process, especially in infants and young children. Throughout the development of the respiratory system, which begins as early as week 4 of gestation, there are significant changes in both the structure and function of respiratory movements. As children grow, the pattern of breathing transitions from predominantly abdominal (diaphragmatic) breathing to chest breathing, with the diaphragm and other respiratory muscles facilitating this process. The full complement of functional alveoli is not present until around the age of 8, indicating that significant development of the respiratory system continues into early childhood. Prior to this age, the abdomen is observed to move more noticeably during respiration. Notably, fetal breathing movements, which involve contractions of the respiratory muscles causing the fetus to inhale and exhale amniotic fluid, begin around week 20 or 21 of gestation. These movements are essential in preparing the muscles for breathing after birth. By adolescence, the normal respiratory rate of a child is similar to that of an adult, which ranges from 12 to 18 breaths per minute, indicating that the fundamentals of the respiratory system's function have matured. Therefore, it can be deduced that the abdomen moves with breathing until around the age of 8 years, when the respiratory system, including the alveoli, reach more developed states.