The official drinking age in Spain is 18 years old, but it does not matter.
Like other European countries, this law is not very well enforced. In addition, the region of Castile supposedly lowered its drinking age to 16.
In response, the Spanish government has threatened to make a national drinking age of 18, but they have not followed through.
According to the famed Wikipedia, the legal drinking age is 16, but the buying age is 18. Also, only the alcohol vendors can get in legal trouble for selling to minors, not the minors themselves.
When I was in Spain, I was only bothered for my ID at one point, and I said that I did not have it. I was let in anyways. So they do not care, as long as you do not become a stereotypical "drunk tourist" or worse, "drunk American."
So, in conclusion, the drinking age is 18, but really, it is Europe and they don't care that much. To generalize Spain on drinking and Spain drug laws, they are more concerned about the health and safety of users than they are of getting the users in trouble.
So, have fun, stay safe, and drink responsibly no matter what country you are in.