On-chip circuit aging sources like negative bias temperature instability (NBTI), hot-carrier injection (HCI), electromigration, and oxide breakdown are reducing expected chip lifetimes. Using sensors to track the actual aging process is one way to avoid unnecessarily large design margins typically used to handle these aging effects. The paper proposes a sensing scheme that uses sets of reliability sensors capable of accurately tracking NBTI PMOS current degradations across process, temperature, and varying activity factors. A novel NBTI sensor is introduced that can track PMOS current degradations across these variation sources. Results show that a set of 1000 such small sensors can predict chip lifetime to an uncertainty of 7 to 10 percent. Additional simulations show that once choosing the total sensing area, the tradeoff between the number of sensors and the area of one individual sensor minimally impacts lifetime prediction uncertainty.