Provided you are non-US resident:
Always book ahead to get the best interline rates.
Use Alamo .. they have far better deals than anyone else ... up to 40% off retail.
go to www.alamo.com and for travel industry rates specify Rate Code W9 for fully inclusive product and RP for Alamo Gold in the appropriate boxes( Corp ID and rate code ).
For travel industry rates you pay in the US ( at the end of the rental .. credit card imprint taken at start of rental ).

Age 24 is fine for Alamo and you don't normally need an International Licence provided your licence is readable in English ( you don't say where you are based however most EU are OK ).