Car rental age policy: Most countries require you to be about 25 when renting a car. Maximum age limits also vary by country and can be as low as 65. If you're concerned, contact your car rental agency and ask what their age limits are.
Driving over the border: Each agency has their own rules about driving out of the country with their car. Contact your agency to make sure their rules fit your plans. See also: What type of documents do I need to rent a car outside of the United States?
Special requests: Need something extra for your car, such as car seat or GPS? Contact your rental agency, and they will do their best to accommodate your needs.