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 Poland.
Article 15 of The Act on Upbringing in Sobriety and Counteracting Alcoholism prohibits sale or serving of alcohol beverages to anyone under 18 and that if doubt exists as to age, sellers/servers should require ID as proof of age. Drinking in public places, with the exception of designated drinking zones, is prohibited regardless of age.
You can find more legal details and links here:
Wikipedia Legal Drinking Page