Guidelines recommend screening all patients with newly diagnosed colorectal cancer (CRC) for Lynch syndrome (LS). However, the efficiency of universal LS screening in elderly populations has not been well studied. To compare the performance of age-restricted and universal LS screening using reflex mismatch repair (MMR) immunohistochemistry (IHC) of CRC tumors. Retrospective cohort study. A large, diverse, community-based health care system. 3891 persons with newly diagnosed CRC who had LS screening between 2011 and 2016. Diagnostic yield of different LS screening strategies. Sixty-three LS cases (diagnostic yield, 1.62%) were identified by universal screening, with only 5 (7.9%) detected after age 70 years and 1 (1.6%) detected after age 80 years. When all patients with CRC who had universal screening were used as the denominator, 58 LS cases (diagnostic yield, 1.49% [95% CI, 1.13% to 1.92%]) were identified in patients with CRC diagnosed at or before age 70 years, 60 LS cases (diagnostic yield, 1.54% [CI, 1.18% to 1.98%]) were identified in those with CRC diagnosed at or before age 75 years, and 62 LS cases (diagnostic yield, 1.59% [CI, 1.22% to 2.04%]) were identified in those with CRC diagnosed at or before age 80 years. Using 75 years as the upper age limit for screening missed 3 of 63 (4.8%) LS cases but resulted in 1053 (27.1%) fewer cases requiring tumor MMR IHC. Using 80 years as the upper age limit missed 1 of 63 (1.6%) LS cases and resulted in 668 (17.2%) fewer cases requiring tumor MMR IHC. Persons who were eligible for but did not complete germline analysis were excluded from calculations of performance characteristics. The incremental diagnostic yield decreased substantially after age 70 to 75 years. Stopping reflex CRC screening for LS after age 80 years may be reasonable because of very low efficiency, particularly in resource-limited settings, but this merits further investigation. Studies evaluating the effect of diagnosing LS in elderly persons on their family members are needed. Kaiser Permanente Northern California Division of Research.