The legal age in China is 14.
One issue in China is that prostitution is not legal.
In Hong Kong the legal age is 16.
Note: although sex with a girl is legal at 16, buggery with a girl under the age of 21 is a criminal offence with a severe penalty.
Disclaimer: I am not a lawyer and am making this posting for general information and to point out that there's a lot of misinformation being posted here. The consequences of getting it wrong are severe, including severe fines, a criminal record, life imprisonment, or worse. Given that, the risk of being faced with blackmail or other consequences are also high. So if you are considering sex with a young person you must contact a lawyer yourself and ask him if the specific acts you are considering with that person contravene the law or not.
Stay safe, happy, and have lots of fun!
http://en.wikipedia.org/wiki/Ages_of_consent_in_Asia
http://www.avert.org/age-of-consent.htm
http://www.legislation.gov.hk/eng/home.htm