18 is old enough to rent a hotel room in most US cities. However, some hotels may restrict access to rooms with liquor in the minibars/courtesy refrigerators or where there is gambling to those guests 21 and over. And yes, there are some cases in which hotels set the bar at 25 years old. Usually, these are higher end hotels that don't want "spring breaker" overload or want more family style/mature guests.
Personal thought: If I am concerned that a prospective date is not old enough to rent a room I don't believe I'd interested in meeting her. That's part of my own risk management protocol. (See: Lawrence Taylor)