Law is all that needs to be looked at, under 18 but over 15 up to 3 years in prison for the punter
Section 8. Any person who, in order to gratify his or her sexual desire or that of another person, has sexual intercourse with or acts in any other ways against a person over fifteen but not over eighteen years of age in a prostitution establishment, with or without his or her consent, shall be liable to imprisonment for a term of one to three years and to a fine of twenty thousand to sixty thousand baht. If the offence under paragraph one is committed against a child not over fifteen years of age, the offender shall be liable to imprisonment for a term of two to six years and to a fine of forty thousand to one hundred and twenty thousand baht. If the act under paragraph one is committed in relation to one's own spouse, and not to gratify the sexual desire of another person, the offender is not guilty.