5th grade (10 years) and up.
Pairs of students are challenged to build a slow car using a motorized LEGO® kit. Students begin with a bicycle-building activity which introduces the concepts of gearing down and gear ratios. They utilize this knowledge to design a car capable of low speeds. The cars are timed on a course to see how slow they can go, and students are encouraged to modify and alter their design throughout the lesson.
For Educators/ Parents:
See this article from the Journal of Research in Science Teaching: 
Reasoning about structure and function: Children’s conceptions of gears