The presence or absence of menses (amenorrhea) has traditionally been used as the primary measure of fertility.
Premature menopause is defined as cessation of menses before the age of 40 years.
Menopause is defined by the absence of menstrual cycles for 12 consecutive months and average age of menopause in the United States is 51.
Several studies have looked at amenorrhea in survivors of cancer, including survivors of childhood cancer. Fertile Hope has created a table summarizing the risk of amenorrhea with various chemotherapy and radiotherapy regimens. (attach link or table)
A study in Ontario, Canada of adult survivors of childhood cancer found that survivors who received both alkylating agents and abdominal-pelvic radiation were more likely to be postmenopausal than were those who underwent surgery alone. In addition, there was a dose correlatiuon of both radiation and alkylating agent and the risks of menopause and infertility. 
The Childhood Cancer Survivor Study (CCSS) found that the cumulative incidence of nonsurgical premature menopause was higher for survivors than for siblings .
In the Euro2K cohort of adult female survivors of childhood cancer, they found the median age at menopause to be 44 years old (average age of menopause in the United States and Europe is 51), with only 2.1% having nonsurgical premature menopause. 
There is a period of time before menopause when women may be still menstruating but have “decreased ovarian reserve” or decreased fertility due to an ovarian factor. Thus the presence of regular menstrual cycles does not correlate with intact fertility or ovarian reserve in childhood cancer survivors. Larsen et al.  noted diminished ovarian reserve (smaller ovarian volume, lower number of small antral follicles per ovary and lower total number of follicles per ovary in adult childhood cancer survivors with regular menstrual cycles and basal FSH <10 IU/l.