Altering the performance of single transistors and integrated circuits at nominal operating conditions over time, as well as soft errors, are serious reliability issues for integrated CMOS circuits, especially when used in space applications. In principle, the effect of soft errors becomes even more critical if the circuit performance degrades over time. To address this detrimental behavior, the impact of performance degradation due to NBTI on the soft error susceptibility of integrated circuits is analyzed thoroughly. For this, we analyze the critical charge sensitivity of the two-input NAND gate and NOR gate for different operating temperatures at stress times up to 3 years. The results show that the critical charge decreases with the temperature and strongly depends on the input states.Next, we validate the results employing the c17 ISCAS'85 benchmark suite, employing the PTM model with the HSPICE to estimate the soft error at the sensitive nodes. The critical charge is observed to be sensitive to the selected supply voltage and device temperature and thus provides a good measure for the soft error susceptibility with respect to NBTI at various operation conditions.