Ambient air pollution is a well-established risk factor for chronic diseases, but its impact on disease onset age remains unclear. This study systematically evaluated the acceleration effect of air pollutants on the onset of 78 chronic diseases using over 900,000 hospitalization records from 396,000 UK Biobank participants. Both particulate matter and nitrogen oxides were associated with accelerated onset of 46 out of 78 diseases (9 cardiovascular diseases, 7 respiratory diseases, 14 psychological/neurological disorders, 3 digestive diseases, 2 cancers, and 11 other chronic diseases). Significant associations including those for common chronic diseases were observed. Each interquartile range (IQR) increase in PM was strongly associated with a 0.93% (95% CI-0.86%, 1.00%) decrease in age at onset (AAO) of hypertension. Similarly, NO was associated with a 0.96% (95% CI-0.82%, 1.09%) decrease in AAO of COPD, PM with a 0.95% (95% CI-0.81%, 1.09%) decrease in AAO of diabetes, and NO with a 0.88% (95% CI-0.77%, 1.00%) decrease in AAO of dementia. Notably, we observed that neurological/psychological disorders were observed to be mostly affected, including schizophrenia, dystonia, polyneuropathies, and migraine, with 1 ~ 3% reduction in the AAO. On a population level, PM overexposure (exceeding the WHO guideline of 5 μg/m) accounted for 539,320 person-years of accelerated AAO across 78 chronic conditions, with hypertension (18.10%), asthma (6.03%), and diabetes (5.39%) contributing the most. This study provides the first evidence that air pollutants could accelerate onset of common chronic diseases. Findings highlight the urgent need for measures to improve air quality to slow progression of disease development.