To find specifically what Piaget said about Cognitive Development in Adulthood, the key materials to start with are:

1) Piaget, J. (1972). Intellectual evolution from adolescence to adulthood. Human Development, 15, 1-12.

2) Fischer, K. W., Yan, Z., & Stewart, J. (2003). Adult cognitive development: Dynamics in the developmental web. In J. Valsiner & K. Connolly (Eds.), Handbook of developmental psychology (pp. 491-516). Thousand Oaks, CA: Sage.

3) Commons, M. L. & Richards, F. A. Four postformal
stages In J. Demick (Ed.), Handbook of adult development. New York: Plenum.