Well, actually not all motor vehicle operators are technically adults. You can get your permit @ 15 years of age and license at 16. Most places you're not adult enough to buy porn or smokes or have sex with people 18 or older 'til you're 18. You're not adult enough to buy booze 'til you're 21.

But still an important point. Maybe some age restrictions instead of transport mode restrictions on the drive thrus? Show ID for your Taco Bell Grande? Should be after curfew when the dining room closes in many communities anyways so call the cops on youngsters that roll or stroll up.