Well, it should be your preference; you can’t make a mistake. Enter the one that works for you.
Maybe she doesn’t give a damn about US age requirements, many teens and adults do drugs and that is illegal for any age.
it can't be at our preference because some elements in the story mentioned after the age's choice can make them incoherent.
For exemple, Abbi is to young to work and go to school, and mc show an old picture of her in american idol, (minimal age 15) ,now she has to be older but since she still in school and to young to work she cant be more than 18.
Abbi also mentions that her first kiss 3 years ago and that she was maybe the same age as Isi is now. so at least 3 years separate them, so Isi have to be 14-15 based on Abbi's age.
for Mickey, mc said that she have to chose a college and she's older than Abbi.
we also have a picture of Mickey a few years back (so 2 or more) with an already adult body, and since she said that when she had Isi's age, she was flat as a plank, she have to be older than Isi is now on that picture but also older than Abbi was in her old picture , so 18+ on that picture and at least 2 years more now so 20years or more.
Her father is supposed to care about his daughters, so if she wasn't old enough to drink, it wouldn't make much sense for him to let her have her stash of booze at home and letting her drink like that for 1 year.
Mc is also supposed to be 36 years old but if Mickey is min 20-21, Abbi 17-18, and Isi 14-15 that would mean that he had his first daughter at 15 so i really think that you should change mc age to 40+ for it to be a little more realistic .(and it would fit better with his look ^^)