Long-term physical aging and plasticization, two mobility-based phenomena that are counterintuitive in the context of "rigid" polymers of intrinsic microporosity (PIMs), were evaluated using pure- and mixed-gas permeation data for representative ladder and semiladder PIMs. PLMs between 1 and 4 years old retained from 10- to 1000-fold higher H-2 and O-2 permeabilities than commercial membrane materials with similar or higher selectivities. A triptycene-based ladder polymer (TPIM-1) exhibited very large selectivity gains outweighing permeability losses after 780 days, resulting in unprecedented performance for O-2/N-2 (P(O-2) = 61 Barrer, alpha(O-2/N-2) = 8.6) and H-2/N-2 (P(H-2) = 1105 Barrer, alpha(H-2/N-2) = 156) separations. Interestingly, TPIM-1 aged more and faster than its more flexible counterpart, PIM-1, which exhibited P(O-2) = 317 Barrer and alpha(O-2/N-2) = 5.0 at 1380 days. Additionally, the more "rigid" TPIM-1 plasticized more significantly than PIM-1 (i.e., TPIM-1 endured similar to 93% increases in mixed-gas CH4 permeability over pure-gas values compared to similar to 60% for PIM-1). A flexible 9,10-bridgehead (i.e., TPIM-2) mitigated the enhancements induced by physical aging but reduced plasticization. Importantly, intoa-chain rigidity alone, without consideration of chain architecture and ultra-microporosity, is insufficient for designing aging- and plasticization-resistant gas separation membranes with high permeability and high selectivity