Background and Aims Age is closely related to the incidence, biological behavior and prognosis of breast cancer, and is an important reference factor for clinical decision-making. Different ages reflect different physiological states of the patients. Since the absence of hormone receptors in triple-negative breast cancer (TNBC) is sex hormone-independent, whether age affects the treatment decisions and prognosis of these patients remains to be studied. This study was conducted to investigate the relationship between age at diagnosis and clinicopathologic features, treatment strategies, and prognosis of TNBC patients.Methods The data of TNBC patients diagnosed as stage I to III during 2010 to 2016 were extracted from the SEER database. Patients were divided into five groups defined as 18-39 years, 40-49 years, 50-59 years, 60-69 years, and ≥70 years group, according to the age at diagnosis. Comparisons of clinicopathologic features and treatments were performed among different age-groups. Multivariate Cox proportional risk model was used to analyze the relationship between age at diagnosis and breast cancer specific survival (BCSS), and the hazard ratio (HR) with corresponding 95% confidence interval (CI) was calculated.Results A total of 30 576 TNBC patients with a median age of 57 years (IQR: 48-67 years) met the criteria were enrolled in the final analysis. Among them, 3 007 cases (9.83%) aged from 18 to 39 years old, 6 071 cases (19.86%) aged from 40 to 49 years old, 8 097 cases (26.48%) aged from 50 to 59 years old, 7 176 cases (23.47%) aged from 60 to 69 years old and 6 225 cases (20.36%) aged ≥70 years old. The distributions of diagnosis year, race, marital status, TNM stage, pathological type, histological grade, surgical treatment and radiotherapy or chemotherapy had significant difference among groups (all P<0.05). With the increase of age at diagnosis, the T stage and N stage were declined, histological grade was improved, and the rates of mastectomy and chemotherapy were reduced. The median follow-up was 32 months (IQR: 15-54 months), and breast cancer-related death occurred in 3 482 cases (11.39%). The BCSS rate was statistically different among different age groups (P<0.001). The univariate Cox proportional analysis showed that the age at diagnosis, race, marital status, T stage, lymph node stage, TNM stage, pathological type, histological grade, surgical treatment and radiotherapy were significantly associated with BCSS (all P<0.05), but no significant connection was observed between the chemotherapy and BCSS (P=0.284). The multivariate Cox analysis showed that patients with the age of 18-39 years (HR=1.00, 95% CI=0.88-1.13, P=0.990), 40-49 years (HR=0.95, 95% CI=0.85-1.06, P=0.330), and 50-59 years (HR=1.03, 95% CI=0.93-1.14, P=0.597) had a similar BCSS with patients aged 60-69 years, while patients aged ≥70 years had a worse BCSS than those aged 60-69 years (HR=1.56, 95% CI=1.41-1.74, P<0.001). The relationship between age at diagnosis and BCSS was similar in subgroup patients with different years of diagnosis, TNM stages, surgical treatment, radiation status, and chemotherapy status.Conclusion Age at diagnosis is significantly associated with the prognosis of TNBC, elder age (≥70 years old) is an independent poor prognostic factor, and the survival is similar among patients aged from 18 to 69 years old.