When humans are born, we're little people called "babies". We are biologically wired to grow into toddlers, adolescents, teenagers and then adults. We are no longer babies because we have gotten taller, our brains have developed to full potential and we can take care of ourselves. It's normal to change from a baby to an adult.
When we're in our mama's belly, we were assigned a gender. Our gender can never change because we cannot change our sex chromosomes no matter how we change our outward appearance.