Transcranial magnetic stimulation (TMS) is non-invasive brain stimulation procedure adopting the intense magnetic pulses to induce an electric field in neuronal tissue, modulating cortical excitability level. Computational modelling and simulation have played a major role in the development of many TMS therapeutic applications for adults such as Major Depression Disorder (MDD) treatment. However, TMS is still being investigated in pediatric and geriatric for numerous clinical applications and conditions due to physiological differences from adults. Studies of TMS induced fields’ effect comparison were usually limited to two age groups Child/Adult or Adult/Elderly, our study included a comparison of three widely spread age groups Child, Adult and Elderly. This study aimed to simulate the strength and distribution of TMS induced electric fields in an MRI base head model of different age groups. Child, Adult and Elderly head MRI images were used. Individualized finite-element method modelling of the TMS induced electric field at the head vertex (Cz) was performed. The profound outcome from the work displays a notable higher TMS induced electric field and larger field expansion in the Grey and White matters of Child and Elderly samples in compare to Adult. The head model for these three groups shows an obvious difference in the white and grey matters of the brain composition. The underdevelopment brain size for children, and shrinking of brain composition for elderlies are very similar. This important point has explained the electric field strength on the brain. The usefulness of the TMS modelling effects on the brain for different age groups may serve patients better for a wider age range.