Purpose To study the relation between the presence of axillary lymph node (LN) involvement and age in breast cancer. Patients and Methods The breast cancer database of the University Hospitals Leuven contains complete data on 2,227 patients with early breast cancer consecutively treated between 2000 and 2005. A multivariate piecewise logistic regression model was used to analyze LN involvement in relation to age at diagnosis. A similar analysis was then performed on a large, independent, population-based database from the Eindhoven Cancer Registry to investigate whether the effects of the Leuven model could be replicated. Results We observed a piecewise effect of age. That is, women up to 70 years of age were less likely to have positive LNs with increasing age (odds ratio per 10-year increase, 0.87). In contrast, older women were more likely to have positive LNs with increasing age. However, for older women, the effect of age interacted with tumor size (P = .0044), suggesting that increasing age is associated with increased risk of LN involvement, mainly in small tumors. These findings were replicated in the Eindhoven Cancer Registry database. Conclusion Axillary LN involvement varies with age at diagnosis; its probability decreases with increasing age up to the age of approximately 70 years, but increases again thereafter. However, this increase is mainly seen in smaller tumors and suggests a different behavior of small breast cancers in older adult patients. We hypothesize that decreased immune defense mechanisms, related with aging, may play a role in earlier invasion into LNs.