BACKGROUND: Neuromodulatory effects of transcranial direct current stimulation (tDCS) in older humans have shown heterogeneous results, possibly due to sub-optimal stimulation protocols associated with limited knowledge about optimized stimulation parameters in this age group. We systematically explored the association between the stimulation dosage of cathodal tDCS and induced after-effects on motor cortex excitability in the elderly. METHOD: Thirty-nine healthy volunteers in two age groups, namely Pre-Elderly (50-65 years) and Elderly (66-80 years), participated in the study. Ten sessions of cathodal tDCS, with a combination of four intensities (1, 2, 3 mA and sham) and three durations (15, 20, 30 min) were conducted over the M1 in each participant. Cortical excitability changes were monitored with TMS-induced motor evoked potentials (MEPs) for up to 2 h after stimulation. RESULTS: Motor cortex excitability was reduced by cathodal stimulation intensities of 1 and 3 mA in both age groups, in accordance with results observed in the younger age groups of previous studies. For the 2 mA stimulation condition, an age-dependent conversion of plasticity into a stimulation duration-dependent excitability enhancement was observed in the Pre-Elderly group, whereas in the Elderly group, LTD-like plasticity was preserved, or abolished, depending on stimulation duration. CONCLUSION: The LTD-like plasticity effects induced by cathodal tDCS originally described in young adults are also observable in older humans, but non-linearities of the resulting plasticity were partially preserved only in the Pre-Elderly, but not the Elderly group. These results aid in understanding age-dependent plasticity dynamics in humans, and to define more efficient tDCS protocols in the aging brain.