Well.. hmm, you're an adult if you're 18, in Croatia. You can drink and you can smoke. Some clubs have an age limit of 21 though. Check in advance if you go.
In Croatia people over 16 must have some form of ID with them at all times, that's the law. If you're a foreigner that would be your passport.
Hmm... what else... I don't know, that's all that comes to mind right now.