Just a few further thoughts on the above:
In deciding on an actor for Mercutio, there aren't, as I said, any guidelines on his age or race (he was played, famously, by Harold Perrineau as a cross-dressing black man in Baz Luhrmann's filmed production), but you might consider the other characteristics of Mercutio when making your casting choice.
Mercutio is often best portrayed by rather flamboyant actors--he is often interpreted as being in love with Romeo himself, which sometimes means a gay actor is cast, or he is coded as gay in the production. He is also, in many ways, the comedic star of the play, so an actor who is excellent with wordplay but also able to evoke pathos when necessary is required. Mercutio also must serve as the older and wiser influence on Romeo, despite his loud and sometimes dirty wit. The actor you cast should certainly be older than your casting choice for Romeo and have a world-weary aspect—able to convey the suggestion that his jokes convey something deeper.
I would be interested in seeing Jim Carrey as Mercutio, an actor who gained fame as a comic but has also showcased the ability to convincingly evoke audience sympathy in such films as The Truman Show. Another interesting, quite different choice might be Aaron Paul—capable of cutting and quickfire wit but with the air of weary wisdom suggesting there is much beneath the surface. Mercutio is a funny character, but, as with many of Shakespeare's fools, there is far more to him than meets the eye. The actor you choose must be able to convey that.