Tomato (Solanum lycopersicum L.) belongs to the crops that are the most sensitive to continuous lighting (CL). We studied age-dependent changes in the sensitivity of tomato leaves to CL. The leaves exposed to CL from a lag-phase of their growth exhibited pronounced chlorosis with the loss of 30% chlorophyll after 2 weeks. The values of the maximum (Fv/Fm) and actual (ϕII) quantum yields of the PSII photochemical activity were decreased, the photosynthesis rate was suppressed, and the relative electrolyte leakage was enhanced. In contrast, the leaves were less sensitive to CL if they had passed their early growth (lag-phase) under normal light conditions (16-h photoperiod) and encountered the CL as late as in the log-phase. In this case, the chlorophyll content, the photosynthesis rate, and the electrolyte leakage were close to the levels of the control leaves grown at the 16-h photoperiod except for the antioxidant enzymes catalase, ascorbate peroxidase,and guaiacol peroxidase—which were more active. The conclusion was drawn that the age-related changes in the CL-sensitivity are due to the difference in activities of the antioxidant enzymes. In general, the elder plants were less sensitive to CL than the younger ones.