Many times 18 year olds are still naive about things like this, are still convinced that they have to do what an authority figure tells them, may be in a situation where they can't leave or the abuser has some other power over them or the person know that no one will believe them and condemn him for even saying something.  My abuser told me over and over that no one would believe me if I said anything and if I told I would be the one in trouble.  And he was right, I got up the nerve to talk to a priest about it and was told I must have wanted it or it wouldn't have happened, with the implication that I had deduced him.  Others I have talked to said they have had the same experience.   And although I used he and him, these things hold true about abused women and girls as well.