It's illegal in America for anyone under the age of 21 to purchase alcohol. That means if you're writing a scene of 18 year-olds toasting to either graduating high school or starting college, you CAN'T SET IT IN A BAR!
That also means that you shouldn't have 18 year old characters openly drinking alcohol at a restaurant - so watch that mistake. Sure, when this thing is cast your "18 year olds" will be played by people far closer to thirty, but try to demonstrate at least a little commitment to plausibility.
Related to this, if you go to the trouble of your characters having fake IDs please spare us the obligatory scene where either the fake name is absurdly lame or the fake picture barely resembles the lead. Superbad got away with "McLovin'" but you're going to have to top that if you don't want it to feel like a retread.
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