The answer is eighteen.
Under the Minors Protection Law of China, the sale of cigarettes or alcohol to minors is prohibited, and business operators shall put up a sign of no sale of cigarettes or alcohol to minors at a prominent place; where it is difficult to determine whether a person is an adult, he/she shall be required to show his/her identity card. Minors refer to citizens under the age of 18.
Reference:
Minors Protection Law of China (2012): Article 2, 37