Today, the increasing need to store more and more information has resulted in the fact that Static Random Access Memories (SRAMs) occupy the greatest part of a System-on-Chip (SoC). Therefore, SRAM's robustness is considered crucial in order to guarantee the reliability of such SoCs over lifetime. In this context, one of the most important phenomena that degrades Nano-scale SRAMs reliability is related to Negative-Bias Temperature Instability (NBTI), which accelerates memory cells aging. This paper proposes a new approach to detect SRAM aging during system lifetime based on an On-Chip Aging Sensor (OCAS). The OCAS is able to detect any specific aging state of a cell in the SRAM array. The strategy is based on the connection of one OCAS every SRAM column, each periodically performing off-line tests by monitoring the write operations on the SRAM cells in order to detect aging. To prevent the OCAS from aging and from dissipating leakage power, the OCAS circuitry is powered-off during its idle periods. Experimental results demonstrate the sensor's high sensitivity to detect early aging states and therefore, guaranteeing high memory reliability. Finally, the area overhead related to the sensors' insertion is almost negligible.