The effect of plant age and environmental factors (minimum, maximum, relative humidity, and temperatures) on the population dynamics of the tomato leafminer Tuta absoluta (Meyrick) was studied at Giza Governorate, during the summer plantations of the tomato plant Solanum lycopersicum in 2021 and 2022. Tomato leaves were picked and inspected weekly to determine how many generations each living instar (eggs and larvae) had completed. Mines with and without larvae were counted. The population density was higher in the first growing season (2021) than in the second growing season (2022), with mean counts of 31.07 and 27.74 insects per 10 leaves, respectively. This pest was able to attain 3–4 peaks for both eggs and larvae, with the first peak occurring in the third week of March, followed by the third week of April, and the largest peaks occurring at the end of May in both seasons. Studies on the relationships between insect pest populations and environmental variables showed that T. absoluta eggs and environmental variables (minimum and maximum temperatures, relative humidity, and plant age, a biotic factor) throughout two growing seasons in 2021 and 2022 exhibited significant negative correlations but insignificant positive correlations. The population dynamics of T. absoluta (egg and larval mines) were affected by environmental conditions and plant age throughout both seasons (2021 and 2022) by 78%–75% and 71%–63%, respectively.