Randomized controlled trials (RCTs) have found no evidence that the storage time of transfused red blood cell (RBC) units affects recipient survival. However, inherent difficulties in conducting RBC transfusion RCTs have prompted critique of their design, analyses, and interpretation. Here, we address these issues by emulating hypothetical randomized trials using large real-world data to further clarify the adverse effects of storage time. We estimated the comparative effect of transfusing exclusively older vs. fresher RBC units on the primary outcome of death, and the secondary composite endpoint of thromboembolic events, or death, using inverse probability weighting. Thresholds were defined as 1, 2, 3, and 4 weeks of storage. A large Danish blood transfusion database from the period 2008-2018 comprising more than 900,000 transfusion events defined the observational data. A total of 89,799 patients receiving more than 340,000 RBC transfusions during 28 days of follow-up met the eligibility criteria. Treatment with RBC units exclusively fresher than 1-, 2-, 3-, and 4-weeks of storage was found to decrease the 28-day recipient mortality with 2.44 percentage points (pp) (0.86pp, 4.02pp), 1.93pp (0.85pp, 3.02pp), 1.06pp (-0.20pp, 2.33pp), and -0.26pp (-1.78pp, 1.25pp) as compared with transfusing exclusively older RBC units, respectively. The 28-day risk differences for the composite endpoint were similar. This study suggests that transfusing exclusively older RBC units stored for more than 1 or 2 weeks increases the 28-day recipient mortality and risk of thromboembolism or death compared with transfusing fresher RBC units.