The age difference you may find weird but that's not for you to judge, you said so yourself that you don't even know how old your roommate is so they could be closer in age than you think. I think you have some case to the ethics of it, generally schools have policies against fraternizing between faculty and students.

If both roommates have to agree to have a guest over then you do have to right to complain about it, but YTA if you immediately report it because it's something you aren't a fan of, did you mention anything about this to her beforehand? Or is this a one time thing you immediately reported? Sounds like they did that because they thought you'd be gone, they probably would have made other plans otherwise. Also, I'd probably look for a new roommate, because that will probably blow up in your face if she decides to never let you have an overnight guest (boyfriend or not) again.