Well, bully for you.
You are no doubt one of those twits who think spies should present accurate calling cards and the CIA should be barred from trafficking with the unsavory.
Thank God you don't get to set the ground rules.
If the perp is the one who starts and engages the agent in the activity then fine, the agent can play along and say he is a 13 year old girl. No problem.
But, if the agent, posing as a 13 year old girl is soliciting conversation then I have a problem with it.
Just as if an undercover agent is on the street corner and a man goes up to him and asks if he has any dope for sale. The man attempting to buy should be arrested. But undercover agents shouldnt be allowed to stand on the street corner and ask passerbys if they want to buy some dope.