Car-sharing programs are reaching one of their final frontiers: college students under the age of 21.

Two of the biggest car-sharing companies, Flexcar and Zipcar, have dropped their age limits to 18 on some campuses. In addition, Flexcar began a pilot program at six schools, including the University of Maryland and three University of California campuses. Students can reserve the Flexcar vehicles as needed, in exchange for hourly fees. Representatives of the schools say that they are optimistic the programs will help alleviate on-campus parking demand, without costing the colleges any money.

"Parking is constrained," says Mark Norman, the CEO of Flexcar, "and where it's not constrained, it can be far away and prohibitively expensive."

Among the contraints are policies at many public universities that preclude undergraduate students from bringing their cars to school.

The latest car-sharing efforts follow on the heels of car-sharing programs for older drivers--from graduate students to professors and staff--at many campuses. In these programs, undergrads usually were excluded. Many undergrads don't require vehicles on a daily basis since they live close to classrooms. Occasionally, however, they need a way to get off campus.

Car-sharing companies haven't been chomping at the bit to include undergrads, though, because of concerns about the cost of insurance for younger drivers. Flexcar got around this issue by requiring students under 21 to not only pay a $250 security deposit but also provide parental permission and proof of insurance, which is often through their parents' plans.