Arrow's impossibility theorem was set out in his PhD thesis, Social choice and individual values. It shows the impossibility of designing rules for social decision making that obey all of a number of 'reasonable' criteria. It is often known as Arrow's paradox.
Working with Gerard Debreu[?] (who won the Nobel prize for this work in 1983), Arrow produced the first rigorous proof of the existence of a market clearing equilibrium, given certain restrictive assumptions. See general equilibrium.
... is spread out with 20.6% under the age of 18, 5.0% from 18 to 24, 26.8% from 25 to 44, 27.7% from 45 to 64, and 19.9% who are 65 years of age or older. The median age ...