It seems to me that the phenomenon of wealthier youth aping the culture and mannerisms of poorer (and more populous) groups cannot be merely a modern phenomenon. But perhaps Western culture was more heavily striated, socially, prior to the modern era. Do we have historical examples of wealthier kids hobnobbing with or "acting" as though they were members of the hoi polloi to seem "cool?"
Sorry for being two years late, but Thomas De Quincey (best known for Confessions of an English Opium-Eater) was destitute at his own choosing for a summer in 1802. He writes about in the Confessions.
In a biography of Nelson's I read Ferdinand I of The Sicilies was singled out as being popular with the Naples' underclass (Lazzaroni) and intermingling with them. They were his most loyal supporters at a time when middle and upper classes were willing to collaborate with French.
Perhaps not the definition of "slumming," but Siddhartha Gautama is probably the earliest example of this.