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Description: The differentiation-dedifferentiation hypothesis of general cognitive ability has been widely studied, but comparable research on crystallized intelligence is scarce. To close this gap, we conducted an empirical test of the age differentiation hypothesis of declarative knowledge as proposed in Cattell's investment theory, which predicts that knowledge differentiates into diverse forms after compulsory education ends. Thereto, a cross-sectional sample of 1,629 participants aged 18 to 70 years (M = 45.3) completed a comprehensive knowledge test comprising 120 broadly sampled questions from 12 knowledge domains, as well as a measure of openness. To investigate age-related differences in the level and structure of knowledge, we performed invariance tests in local structural equation models. The results did not provide any evidence for age-related differentiation of declarative knowledge but indicated age-related differences in the mean structure. Higher levels in openness were associated with higher levels in knowledge but not with more differentiated structure of knowledge. Contrary to predictions of the investment theory, our results suggest that declarative knowledge is a highly stable construct across major parts of the adult lifespan.