What changes happen to a teenage girls body when they turn 16?

Depends on puberty. That really depends on a lot of factors and when/if she has completed puberty. Most girls have completed puberty and linear growth by 16.5 years. Average onset of periods is 12ish but may occur as late as 17, and a small percentage of women will have some growth after 19. Body fat increases, lean mass decreases. Periods regulate better. Weight increases slightly. But overall not a ton of changes.
Maturation. A teenage girl's body can go through many changes, and each young woman's body changes at its own rate. Most teenagers at age 16 have begun to menstruate. Linear growth (height) can increase slowly for 2-3 years after the onset of your period. Breast growth continues into the early 20's.