The drinking age is 18, officially. In practice most places have no rules, unless you are obviously under aged and obnoxious too.
Personal possesion and use of pot is mostly ignored. Often the local cops will confiscate what you are carrying and that is about it. So never carry more than what you plan to use. HAving enough pot for, say, 4 joints might be enough for some cops to think you a dealer.
BUT, and this is a big big BUT, foreigners violating any law are seen as source of easy income. Do not make the first move in bribing any one, they will let you know soon enough what they want.
If you do get busted you can expect a miserable time and a lot of expense to get out. Here they first lock you up in preventive detention while charges are considered and prepared. Here is where you start using money, to get out provisionally, or to eat, if you stay in, unless you are willing to stick to the prison diet, which most people do not try to do. Your lawyers will price their services not to the crime but to your wealth, and your family's wealth.
Just be discreet and if you smoke some pot also make sure you have a boozy smell as well, spill some beer on your shirt or something.
EDIT. Don't drive drunk or stoned. The penalty is serious and enforcement is getting serious.