Insanity, YAX.
If a guy still sees an old wrinkled cop chick as a hot teen, you'll agree he needs treatment. The insanity plea will chop down his penalty big time!
But where was this? Did you know that India's top justice has wanted to help a rapist if the rapist would just marry the victim?
Elsewhere around the U.S., there's the mens rea idea that may make one guilty of a crime. So, cops love to plant things, like an entrapment in your example, to see if a guy would have the mind to fall for it.
I don't like cops doing entrapments, partly because they would really be involved in getting the felons guilty. But it's usually used to capture serial felons only. I'm expecting that your suspect is a serial teen sex predator.
But, again, insanity will chop down his guilt. So, too, if he was in India. There, the obligation he would have to serve is pleasing the old wrinkled cop for the rest of her life.