The French and Italians definitely let kids drink at an early age - maybe small glasses of wine. And it wouldn't surprise me if the Germans let their kids drink smalls glasses of beer or similar. Same for the Irish and Scottish. And we Brits? 14 y/o in the pub, downing pints. Proper like! And then we grow up into holiday-makers who go abroad, get absolutely rat-arsed, and annoy the locals.
Makes sense, I suppose. 18 just isn't as much a big deal as 21 is. Plus, with the drinking age being 18 (lower in some European countries, I think) people then get three years practice at University before they are let loose on the world.