Relationship of age to incidence of breast cancer in young women.

The rate of increase with age in the incidence of breast cancer in women was at a maximum at the lowest age that it could reasonably be estimated (in these data, 25 yr). It then declined linearly with age to about 50 years. The rate of increase with age, and its changes with age, were similar in many Western populations and in Japan. The decline with age in the rate of increase in the incidence of breast cancer was arrested at the age of 50 and replaced by rates of change that altered little with age. The premenopausal changes could be reproduced by a breast cancer precursor model with exponential decay of precursor prevalence with time. There was evidence for the existence of such a precursor.