The average age that girls begin wearing bras is age 11 which is when the girl is ready to start wearing what's known as a training bra.
However some girls need to start wearing a training bra as early as age 8 years old.
Girls are all different from each other so if you think your daughter needs to wear a bra by age 8 then be prepared to get her into wearing a training bra even as early as age 8.
But then again I've known some girls that didn't start wearing a bra until as late as age 14.
So there's no real set age as to when a girl will start wearing a bra because it depends on the girls growth and how fast they grow and change.