If you’re under 18 and get caught buying or drinking in public, the police will probably confiscate your booze. They will also probably give you a very disappointed look.
You might get a fine or a warning that your parents will be told. That’s usually worse than any legal punishment. The fear of your mum’s disappointed sigh is a powerful deterrent.
And for the adults who buy booze for minors? That’s a proper fine and a potential criminal record. So don’t be that guy who gets a teenager a double vodka and a packet of crisps.