It is common knowledge that young persons are different from older ones, and not just in the ways they look and behave. They also differ in how they think. This is not simply a matter of being more or less expert at intellectual tasks; often such differences are negligible. Adulthood is a matter of being differently cognitively organized and motivated, especially in relation to time. It's surprising how little these differences have been studied scientifically. This essay is about the fundamentally cognitive ways that older people differ from their younger selves. It's about being grown up.