With no known treatments or vaccine, COVID-19 presents a major threat, particularly to older adults TRANS, who account for the majority of severe illness and deaths MESHD. The age TRANS-related susceptibility is partly explained by increased comorbidities including dementia MESHD dementia HP and type II diabetes. While it is unclear why these diseases MESHD predispose risk, we hypothesize that increased biological age TRANS, rather than chronological age TRANS, may be driving disease MESHD-related trends in COVID-19 severity with age TRANS. To test this hypothesis, we applied our previously validated biological age TRANS measure (PhenoAge) composed of chronological age TRANS and nine clinical chemistry biomarkers to data of 347,751 participants from a large community cohort in the United Kingdom (UK Biobank), recruited between 2006 and 2010. Other data included disease MESHD diagnoses (to 2017), mortality data (to 2020), and the UK national COVID-19 test results (to May 31, 2020). Accelerated aging 10-14 years prior to the start of the COVID-19 pandemic was associated with test positivity (OR=1.15 per 5-year acceleration, 95% CI: 1.08 to 1.21, p=3.2x10-6) and all-cause mortality with test-confirmed COVID-19 (OR=1.25, per 5-year acceleration, 95% CI: 1.09 to 1.44, p=0.002) after adjustment for demographics including current chronological age TRANS and pre-existing diseases MESHD or conditions. The corresponding areas under the curves were 0.669 and 0.803, respectively. Biological aging, as captured by PhenoAge, is a better predictor of COVID-19 severity than chronological age TRANS, and may inform risk stratification initiatives, while also elucidating possible underlying mechanisms, particularly those related to inflammaging.