Since most state laws require a person to be carded when purchasing cigarettes if the individual looks under 40, then it may be sufficient for an officer to ask you.  However, keep in mind, if you refuse to show your identification, you might be considered to have violated other laws, including obstruction of justice or simply lying to an officer.  Probable cause may not be the correct term.  The police may have articulable suspicion and may request certain pieces of the puzzle to determine if you are indeed underage drinking or smoking.  These are part of the health and safety laws of your state.  Probable cause is a higher level of knowledge, think enough to get a warrant.  If you were cited for underage smoking because you showed your identification and are wondering if it was required of you to show your identification, then the answer is yes.