The image below is a schematic flow diagram of a typical oil refinery that depicts the various unit processes and the flow of intermediate product streams that occurs between the inlet crude oil feedstock and the final end products.
In order to check-in into a hotel, the person must be 21 years old.  If the person is younger than 21, then a parent or guardian can  give permission for them to check-in. 1 person found this useful.
As to checking in, I don't see how they can not let the 18 year old check in as long as it is on their credit card. Age is not one of the valid reasons a hotel can refuse to check a guest in in FL.