A hypothesis has been proposed that breast cancer in women is caused in part by events that occur between the age at menarche (the age when menstruation begins) and the age at first childbirth. In particular, the hypothesis is that the risk of breast cancer increases as the length of this time interval increases. If this theory is correct, then an important risk factor for breast cancer is age at first birth. This theory would explain in part why breast-cancer incidence seems to be higher for women in the upper socio-economic groups, because they tend to have their children relatively late.

An international study was set up to test this hypothesis. Breast cancer cases were identified among women in selected hospitals in the United States, Greece, Yugoslavia, Brazil, Taiwan, and Japan.Controls were chosen from women of comparable age who were in the hospital at the same time as the cases but who did not have breast cancer. All women were asked about their age at first birth.....

The set of women were divided into two categories of women whose age at first birth was 29 years or less and women whose age at first birth was 30 or greater. What type of variable is their age? Is age always the same type of variable?