Females begin to ovulate when they reach puberty and their menstrual cycle begins. This could be before they turn 13 or many years after they turn 13. The average cycle is 28 days, but what is normal for an individual female depends on the female. The cycle may not be as regular in the first couple of years.
Teenage girls start to ovulate as soon as they reach puberty. For the first couple of years her menstrual cycle may be shorter or longer than the normal 28 days.