If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in Cambodia.
The Health Ministry drafted a law to regulate the sale and consumption of alcohol in June 2015, with emphasis on establishing a minimum legal purchase age of 21 and preventing underage drinking. The draft law has not yet been enacted. https://www.phnompenhpost.com/national/groups-urge-passing-alcohol-law-be-sped
You can find more legal details and links here:
Wikipedia Legal Drinking Page