If a girl starts kindergarten at the age of 5, she will be 12 years old at the beginning of the seventh grade. This is assuming that she advanced one grade each year.
Most children in the United States begin kindergarten at the age of 5. Although each state determines its own rules for the minimum age of enrollment, most states require that a child must turn 5 by the September month of the year they begin kindergarten. In some states, parents are allowed to wait until the child is 6 years old if they feel she is not ready to begin formal education.