An 18 year old prostitute is extraordinarily likely to be in that situation as a result of a lifetime of abuse and desperation. Taking advantage of that for sexual pleasure, especially at the age of 73, makes you a terrible person.

An old man taking advantage of a young woman is the issue. One year and tiny permutations of the details which change the legal situation do not change the moral situation that in any similar situation, the old man is a indefensible scumbag.

It doesn't matter if it is a hypothetical or not, defending that old man by saying the victim would have presented herself as willing, is a big issue. Not a witch hunt, not mob rule.