235 ILCS 5/6-16.2
A municipality or county may prohibit a licensee or any officer, associate, member, representative, agent, or employee of a licensee from permitting a person under the age of 21 years to enter and remain in that portion of a licensed premises that sells, gives, or delivers alcoholic liquor for consumption on the premises. No prohibition under this Section, however, shall apply to any licensed premises, such as without limitation a restaurant or food shop, where selling, giving, or delivering alcoholic liquor is not the principal business of the licensee at those premises.
In those instances where a person under the age of 21 years is prohibited from entering and remaining on the premises, proof that the defendant-licensee, or his employee or agent, demanded, was shown, and reasonably relied upon adequate written evidence for purposes of entering and remaining on the licensed premises is an affirmative defense in any criminal prosecution therefor or to any proceedings for the suspension or revocation of any license based thereon. It shall not, however, be an affirmative defense if the defendant-licensee , or his agent or employee, accepted the written evidence knowing it to be false or fraudulent.
Adequate written evidence of age and identity of the person is a document issued by a federal, state, county, or municipal government, or subdivision or agency thereof, including, but not limited to, a motor vehicle operator's license, a registration certificate issued under the Federal Selective Service Act, or an identification card issued to a member of the armed forces.
If a false or fraudulent Illinois driver's license or Illinois identification card is presented by a person less than 21 years of age to a licensee or the licensee's agent or employee for the purpose of obtaining entry and remaining on a licensed premises, the law enforcement officer or agency investigating the incident shall, upon the conviction of the person who presented the fraudulent license or identification, make a report of the matter to the Secretary of State on a form provided by the Secretary of State.
235 ILCS 5/6-16.2