You will sign a document when you collect the courtesy car and any such limits will be on it. Just read the document and if you find that there is a limit and you are likely to exceed it, mention it to the person you are dealing with. They wan't your business so it is unlikely they would want to disappoint you. :)
For instance I was 2-3 years under the minimum age limit they have for courtesy car drivers when I went in the first time. Needless, they gave me the car.