It's 18, but like in most Mediterranean destinations / resorts, you are not really going to have any problems if you are younger. they have a far different attitude to booze there than over here:
a) the bar tenders don't worry about losing their licenses, so what to maximise their takings, so will serve anyone
b) its a far less "aggressive" drinking culture, it is sociable, not "let's get comatozed" as seems to be the norm increasingly in the UK. That's true for most of Europe, but Croatia is still quite a family orientated destination, so they don't have too many problems with stupid "Brits on the piss" brigades. In Dubrovnik, for example, its mainly very chic cafe bars, people drinking cocktails and sitting out on the harbour. anyone aiming to put away 20 pints would get served, but if he caused any problem he'd also get the **** kicked out of him by the local coppers too.
if you're going there, I'm sure you'll have a great time. hvar's definitely worth visiting if you're near split, as is brac, rovinj is interesting, dubrovnik v. stylish but pricey now.