that's a tough question.
you have to be 21 in order to book a room.
but, if the adult in the family is booking two rooms, that could still work by listing husband and wife the main person in each room.
i know you can book two rooms with one ap when you have minor children with making more than 5 in a room.
the part of having an 18 yo makes this more of a difficult question for us to answer.
i guess you should call UO and ask them for their policies.