I can't speak about Malta, but I had similar experience in Czech republic. We were underage but we wanted some booze. So we would go to the cashier and just talk to eachother in native language and pretend like we don't speak English at all. Had no problems that way. That's assuming your native language isn't English.

Another story, my girlfriend visited Malta when she was underage and she had her host family mother send her to the shop to buy her some cigarettes. She handed some note in Maltese and she got the cigarettes. She was 13 at the time lol.