We report an investigation of the stability and durability of p- and n-channel polymer thin film transistors in air over a 4-year period. All-polymer p/n complementary inverters fabricated from an n-channel poly(benzobisimidazobenzophenanthroline) (BBL) transistor and a p-channel poly(3-hexylthiophene) (P3HT) transistor showed excellent switching characteristics and a large voltage gain. The electrical parameters (electron mobility, on/off current ratio, and threshold voltage) of the n-channel BBL transistors in air were found to be constant over the 4 years. The performance of the p-channel P3HT transistors deteriorated dramatically after only 2 weeks in air. The excellent stability/durability of the BBL transistors in air is explained by the closely-packed crystalline morphology which creates a kinetic barrier against diffusion of extrinsic molecules and its high electron affinity that provides energetic stability against chemical/electrochemical reactions. The results demonstrate the longest air-stability and durability of non-encapsulated organic electronic devices to date while offering insights for the design of more environmentally rugged organic semiconductors.