This is inevitable, but they cannot buy it themselves in a store. The owner risks having his store closed and having to pay a large fine if he sells alcohol to minors.
Theoretically you're right, but from my experience a lot of cashiers don't care if you're 18 or not. Some do and ask for your ID, but especially in the more rural areas it generally isn't a big problem to buy harder Alkohol. Of course the shop is risking a fine but I haven't heard about any sort of market getting fined for selling Alkohol to minors.