BACKGROUND: Few studies have attempted to use clinical and laboratory parameters to stratify COVID-19 patients with severe versus non-severe initial disease and evaluate age-specific differences in developing multiple different COVID-19-associated disease outcomes. METHODS: A retrospective cohort included patients from the electronic health database of Hong Kong Hospital Authority between 1 January 2022 and 15 August 2022 until 15 November 2022. The cohort was divided into three cohorts by age (≤ 40, 41-64, and ≥ 65 years old). Each age cohort was stratified into four groups: (1) COVID-19 critically exposed group (ICU admission, mechanical ventilation support, CRP > 80 mg/L, or D-dimer > 2 g/mL), (2) severely exposed group (CRP 30-80 mg/L, D-dimer 0.5-2 g/mL, or CT value < 20), (3) mildly-moderately exposed group (COVID-19 positive-tested but not fulfilling the criteria for the aforementioned critically and severely exposed groups), and (4) unexposed group (without COVID-19). The characteristics between groups were adjusted with propensity score-based marginal mean weighting through stratification. Cox regression was conducted to determine the association of COVID-19 disease severity with disease outcomes and mortality in the acute and post-acute phase (< 30 and ≥ 30 days from COVID-19 infection) in each age group. RESULTS: A total of 286,114, 320,304 and 194,227 patients with mild-moderate COVID-19 infection; 18,419, 23,678 and 31,505 patients with severe COVID-19 infection; 1,168, 2,261 and 10,178 patients with critical COVID-19 infection, and 1,143,510, 1,369,365 and 1,012,177 uninfected people were identified in aged ≤ 40, 40-64, and ≥ 65 groups, respectively. Compared to the unexposed group, a general trend tending towards an increase in risks of multiple different disease outcomes as COVID-19 disease severity increases, with advancing age, was identified in both the acute and post-acute phases. Notably, the mildly-moderately exposed group were associated with either insignificant risks (aged ≤ 40) or the lowest risks (aged > 40) for the disease outcomes in the acute phase of infection (e.g., mortality risk HR (aged ≤ 40): 1.0 (95%CI: 0.5,2.0), HR (aged 41-64): 2.1 (95%CI: 1.8, 2.6), HR (aged > 65): 4.8 (95%CI: 4.6, 5.1)); while in the post-acute phase, these risks were largely insignificant in those aged < 65, remaining significant only in the elderly (age ≥ 65) (e.g., mortality risk HR (aged ≤ 40): 0.8 (95%CI: (0.5, 1.0)), HR (aged 41-64): 1.1 (95%CI: 1.0,1.2), HR (aged > 65): 1.5 (95%CI: 1.5,1.6)). Fully vaccinated patients were associated with lower risks of disease outcomes than those receiving less than two doses of vaccination. CONCLUSIONS: The risk of multiple different disease outcomes in both acute and post-acute phases increased significantly with the increasing severity of acute COVID-19 illness, specifically among the elderly. Moreover, future studies could improve by risk-stratifying patients based on universally accepted thresholds for clinical parameters, particularly biomarkers, using biological evidence from immunological studies.