An online sting using undercover officers posing as boys and girls led to the arrest of 21 men, many of whom showed up at a Mercer County residence and train station to meet the fictitious teens for sex, authorities said Thursday. Those charged included four registered sex offenders. Half of the arrests were made from April 7 to April 10 with the rest made in recent weeks with the help of the U.S. Marshals Service. “The social platforms that were used in these encounters included Kik, Skout, Whisper, Grindr, GROWLr, and MeetMe,” according to the Mercer County Prosecutor’s Office. “Once chattin…