Purpose: To evaluate the impact of chemotherapy on subjective cognitive functioning according to age in a large cohort of breast cancer patients. Methods: Within the UMBRELLA cohort, 715 patients with early-stage primary invasive breast cancer (T1-3N0-1M0) were selected. Subjective cognitive function was assessed by means of the EORTC QLQ-C30 up to 24 months and compared between patients treated with and without chemotherapy, for three different age strata (355 patients < 55 years, 240 patients aged 55–65 years, and 120 patients > 65 years). Differences between chemotherapy and non-chemotherapy patients by age at different time points were assessed by linear mixed-effect models correcting for age, tumor stage, educational level, endocrine therapy, anxiety, and depression. Results: In total, 979 patients from the UMBRELLA cohort were included, of which 715 (73%) responded to baseline and at least one follow-up questionnaire. Questionnaire response rates ranged between 92 and 70%. The proportion of patients treated with chemotherapy decreased with age: 64% (n = 277) in patients < 55 years, 45% (n = 107) in patients 55–65 years, and 23% (n = 27) in patients > 65 years. Chemotherapy was associated with reduced subjective cognitive functioning. The impact of chemotherapy on subjective cognitive function was most pronounced in patients < 55 years, followed by those between 55 and 65 years. In the youngest age groups, patients treated with chemotherapy had significantly lower cognitive functioning up to 24 months. In women over 65 years, subjective cognitive functioning was comparable between patients treated with and without chemotherapy. Conclusion: This study confirms that chemotherapy is associated with impaired subjective self-reported cognitive functioning in breast cancer patients, and the effect persists at least up to 2 years after diagnosis. The impact of chemotherapy on self-reported cognitive functioning in the first 24 months is most pronounced in younger patients, especially those under 55 years of age.