Why do large rental car companies not rent in the 18-21 year old age bracket?
This was selected as Best Answer John, it is due largely to their circumstances at that age. They generally carry low limits on their insurance polices, and do not have the proper, if any credit, as well as a high accident ratio If this is a situation due to a motor vehicle accident, most large rental companies will work with the Insurance company to provide a rental. This is a business decision by the rental company, so your question is best posed to them.
