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We reported that a minority of patients with low-risk papillary microcarcinoma of the thyroid showed disease progression during active surveillance and that older patients had significantly lower disease progression rates than younger patients. Here, we estimated lifetime (≤85 years old) probabilities of disease progression during active surveillance according to the age at presentation based on age decade-specific disease progression rates.From 1993–2013, 1,211 low-risk papillary microcarcinoma patients aged 20–79 years underwent active surveillance at Kuma Hospital. We calculated the disease progression rate at the 10-year point of active surveillance for each age-decade group (20s to 70s) with the Kaplan-Meier method. The lifetime disease progression probability for each age group was calculated as (1 − cumulative probability of progression-free survival calculated with age decade-specific disease progression rates) until the patients reached their 80s (i.e., 85 years on average).The age decade-specific disease progression rates at 10 years of active surveillance were 36.9% (20s), 13.5% (30s), 14.5% (40s), 5.6% (50s), 6.6% (60s), and 3.5% (70s); the respective lifetime disease progression probabilities were 60.3%, 37.1%, 27.3%, 14.9%, 9.9% and 3.5% according to the age at presentation.The estimated lifetime disease progression probabilities of papillary microcarcinoma during active surveillance vary greatly according to the age at presentation.