Graffiti has been found in Pompeii [Link] including pictures and text. Some of it reads like it's current. The modern era of graffiti started in the US with a bike messenger in NYC in the late 1960s, who used the marker incribed tag "Taki 183". He lived on 183rd street, and became famous after an article in the NY Times in 1971. He is still alive, named Demetrius, and has come out of hiding.
Source: NYTimes [Link]. Video [Link]
Of course, he had a predecessor, an unknown that used "Julio 124".