It's illegal to buy or drink alcohol if you're less than 21.
The main beers available in Saipan establishments are Budweiser and Miller products, usually sold in bottles only. However, a few places do serve Fosters or Victoria Bitter on tap, and a few have Miller Lite on tap as well. Other brands widely sold are San Miguel Philippines, Tsingtao China, Sapporo Japan, bottled in Canada, and Corona Mexico. Plenty of stores on Saipan have low-priced, good quality wine available, and there are plenty of harder drinks as well as mixers available everywhere.
Tap water on Saipan is too salty to drink.