A 17-YEAR-OLD youth was sent to a detention centre yesterday for having sex with an underage girl. Ong Hang-shun pleaded guilty in the District Court before Deputy Judge Andree-Wiltens to five counts of having unlawful sexual intercourse with a girl under 13 between September 6 and 29, last year. The judge said the charges were serious but the girl was partly responsible for the offences. The court was told that the girl, then 12 years and 11 months old, went with friends to Tsim Sha Tsui East on August 27 last year. The group befriended Ong and another person and exchanged phone numbers. The first offence happened 10 days later, when Ong took the teenage girl to his Tsz Wan Shan Estate home where they had sex. It was followed by four similar offences in three other locations.