Illinois law about drinking alcoholic beverages inside a moving vehicle says that:

The manual lets police cite both the driver and any passenger, not just the person doing the drinking. This answer reads the ban as a personal-conduct offense aimed at the drinker, treating the citation the way a public-intoxication ticket works instead of a shared-responsibility rule inside a passenger car.

The prohibition bars anyone in the covered vehicle from drinking; a passenger's open drink can still bring citations for both of you. This answer confuses the open-container rule with the DUI rule, assuming the driver's own sobriety is the only alcohol test that matters inside the car.

Being 21 covers who may possess alcohol at all, but in an ordinary passenger vehicle adult passengers still can't drink. The manual exempts only chartered nonschool buses, motor homes, mini motor homes, and limousines. This answer applies the 21-and-over drinking-age rule to the in-vehicle prohibition, assuming that being of legal drinking age also grants the right to drink in a passenger car.