drinking age in the 50's/60's was 21. I believe she is supposed to be around 19 or 20 by the end of the flick and the year is around 1968. It wasn't till the 70's the drinking age was dropped to 18. In the late 60's some states bounced around from 20 to 21yo
I'm guessing She had money and she looked mature/older than girls her age so getting booze wasn't a big deal for her