There was a question about this a while back. I assumed that it was 16 as most of Spain is, but an ABer who lives in the Canaries said that the local law is 18 but its not actually enforced. I've never been asked for ID abroad, as long as you look 16 or over...
sammd....I've holidayed in most of the Canary Islands and from what I've seen the laws on drinking are either non-existant or totally unenforced. The worst of the lot was the area around "Veronicas" in Playa de las Americas, Tenerife where absolutely anything immaginable [not just drinking] was allowed to go on. A pity, because very much like Ibiza, the whole island gets an undeserved reputation because of the antics of a minority.
ianess - I know what you mean about the non-existent/unenforced drinking laws. I was also unimpressed by Playa de las Americas in Tenerife and would never go back. Although i didn't see what you're talking about, there were prostitutes lined up along the pavement near the clubs and loads of pick pockets (all black - no offence to anyone) waiting to prey on unsuspecting tipsy tourists. One even knocked my male friend out when he was walking between clubs on his own and stole his wallet. I'd never go back.