Babies do breathe using their abdominal muscles, however the normal rate for an infant, up to 6 months of age is 30 to 60 breaths per minute. The adult respiratory rate kicks in by the early teenage years.
Why most people change to shallow upper respiratory type breathing is unknown, however I'd hazard a guess and say that sitting relatively still in classrooms, along with a general increase in sedentary lifestyles, has played a significant role.
