Some hotels sometimes require you to be 21 because of some restrictions, most of the hotels I've stayed at lately allows 18 or older to stay. Just to make sure your ok, the best thing to do is check the hotel policy. Plus most hotels a room with 1 bed will hold 2 to 3 people. Usually a 2 bed or suite will hold up to 4 to 5 depending on the hotel and the room, sometimes you got to be careful because some hotels might charge fees when bring in extra people in a room. Also the checking in part is fine, usually if you made a reservation online, they would just ask for ID, Credit Card, and sometimes I've would bring my hotel confirmation just in case. But lately past few years I've stayed in a hotel on my own so I've didn't have too much of a problem.
I got a little carried away with this post. XD