WASHINGTON - A sun shines on Dan Yu's back, alongside a swimming koi fish. A tree soon may grow on his arm. "Your body's an empty canvas, so you almost want to continue to add to it," said Yu, 28, as he showed off his tattoos. A generation or two ago, Yu's tattoos — to say nothing of his pierced nose — probably would have placed him in a select company of soldiers, sailors, bikers and carnival workers. But no longer: The American University employee is among about 36 percent of Americans age 18 to 29 with at least one...