Lego kits help kids develop spacial reasoning, learn to patiently follow detailed directions, and grasp some basics about mechanics. They provide a great opportunity for parents and their children especially at younger ages to work on a project together. And kids end up with fun toys.
Various themed lego kits (dinosaurs, space exploration) also present an easy segue to lessons about science.
Although some of the kits recommend ages 7–12, my son started working with them (with adult supervision) at age four. Just be sure kids have gotten over putting small objects in their mouths! Use at your own risk.