A mistake of fact as to age is an affirmative defense against a charge of buying alcohol for an underage person who later causes a serious accident, but the prosecution is not required to prove knowledge of the person’s age as part of the substantive offense. The defendant here was convicted of a violation of Business and Professions Code section 25658, subdivision (c), after the trial court granted the prosecution’s motion to exclude evidence that the defendant did not know that the minor who caused the accident was under 21 when he served the minor beer at his house. Noting that section 25658(c) does not explicitly require knowledge that the person being served alcohol is under 21, the Court held that the statute goes beyond the so-called “shoulder-tap” scenario and encompasses any situation in which an adult buys alcohol specifically for an underage person, and that the People need not prove that the buyer knew the underage person was not yet 21. However, the Court held that mistake of fact as to the person’s age could be asserted as an affirmative defense to the charge, and accordingly reversed because defendant was not allowed to present evidence on that issue.