The legal drinking age in Quebec is 18. You should bring a photo ID card to prove that you are at least 18.
Beer and wine is sold in dépanneurs (convenience stores) and supermarkets. SAQ stores are government owned liquor stores that sell spirits as well as wine and beer. Alcohol cannot be sold after 11pm. Of course, you can buy alcohol until last call (3am in Montreal) in bars and restaurants as long as you drink it there.