It would be unusual for any bars to ask for ID unless they wanted to make an excuse not to let you in,ie: if you wobbled towards the door.
Clubs may do but again if you arrive fairly early and look the part you should have no trouble anywhere in Ibiza,have a great time,John.

P.S.Most establishments would not expect you to carry your passport around with you but the police will as its the law.They will usually be quite happy with a photo copy so take one with you as it might save you a night in a cell awaiting confirmation of who you say you are........

The legal drinking age in Spain is currently 16, however apparently by some cruel twist you are only allowed to actually buy it at 18. So providing your 18 old friends get all the drinks in, you will be perfectly legal to drink them and you'll be quids in!

Incidently, none of the above really matters as Spanish culture has a very liberal view of alchol, so unless you look about 12, you probably won't have any problems.

You'll also be allowed to get naked and dance on the tables if you like!