BACKGROUND: The novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, also called 2019-nCoV) causes different morbidity risks to individuals in different age groups. This study attempts to quantify the age-specific transmissibility using a mathematical model. METHODS: An epidemiological model with five compartments (susceptible-exposed-symptomatic-asymptomatic-recovered/removed [SEIAR]) was developed based on observed transmission features. Coronavirus disease 2019 (COVID-19) cases were divided into four age groups: group 1, those </= 14 years old; group 2, those 15 to 44 years old; group 3, those 45 to 64 years old; and group 4, those >/= 65 years old. The model was initially based on cases (including imported cases and secondary cases) collected in Hunan Province from January 5 to February 19, 2020. Another dataset, from Jilin Province, was used to test the model. RESULTS: The age-specific SEIAR model fitted the data well in each age group (P < 0.001). In Hunan Province, the highest transmissibility was from age group 4 to 3 (median: beta43 = 7.71 x 10(- 9); SAR43 = 3.86 x 10(- 8)), followed by group 3 to 4 (median: beta34 = 3.07 x 10(- 9); SAR34 = 1.53 x 10(- 8)), group 2 to 2 (median: beta22 = 1.24 x 10(- 9); SAR22 = 6.21 x 10(- 9)), and group 3 to 1 (median: beta31 = 4.10 x 10(- 10); SAR31 = 2.08 x 10(- 9)). The lowest transmissibility was from age group 3 to 3 (median: beta33 = 1.64 x 10(- 19); SAR33 = 8.19 x 10(- 19)), followed by group 4 to 4 (median: beta44 = 3.66 x 10(- 17); SAR44 = 1.83 x 10(- 16)), group 3 to 2 (median: beta32 = 1.21 x 10(- 16); SAR32 = 6.06 x 10(- 16)), and group 1 to 4 (median: beta14 = 7.20 x 10(- 14); SAR14 = 3.60 x 10(- 13)). In Jilin Province, the highest transmissibility occurred from age group 4 to 4 (median: beta43 = 4.27 x 10(- 8); SAR43 = 2.13 x 10(- 7)), followed by group 3 to 4 (median: beta34 = 1.81 x 10(- 8); SAR34 = 9.03 x 10(- 8)). CONCLUSIONS: SARS-CoV-2 exhibits high transmissibility between middle-aged (45 to 64 years old) and elderly (>/= 65 years old) people. Children (</= 14 years old) have very low susceptibility to COVID-19. This study will improve our understanding of the transmission feature of SARS-CoV-2 in different age groups and suggest the most prevention measures should be applied to middle-aged and elderly people.