Final answer:
Premature babies can usually breathe on their own by 28 weeks due to mature alveoli, but full respiratory development continues into childhood.
Step-by-step explanation:
A baby born prematurely can typically breathe on its own at around 28 weeks of gestational age because enough alveoli have matured. However, alveoli continue to develop and mature throughout childhood, with a full complement not present until about 8 years of age.
Prior to 28 weeks, crucial structures such as pulmonary surfactant and adequate surfaces for gas exchange may not be sufficiently developed. Fetal breathing movements begin around week 20, which serve to stimulate the lungs and muscles involved in breathing, though true respiration doesn't begin until after birth.
The development of the respiratory system begins at about week 4 of gestation. By week 28, enough alveoli have matured that a baby born prematurely at this time can usually breathe on its own.