The technical side:
The legal age-limit to buy alcohol is now 18, tobacco is 16
Officially you have to be 18 to get into the clubs.
It is illegal to drink alcohol walking on the streets.

Possession of drugs:
The situation in Spain is a confused one. Although in 1992 a law has strictly regulated the use of drugs in public places (no smoking spliffs on the beach), it is not considered a crime, and therefore there can be no jail sentences. In Spain personal possession of any drug is not a criminal offence. However, read the following extract from The Ibiza Sun:
'One British youth was sentenced this week to 3 years imprisonment and heavily fined after being caught with 6 ecstasy tablets and 20.000 Pts in cash' (October 2000)

The line between 'personal' and 'dealing' is a very thin one. Be sensible.