“Aging Magician,” a new multimedia theater work about the journey of an elderly man to a Coney Island magic show, will be presented at the Watermill Center on Saturday at 4 p.m. With music composed by Paola Prestini, libretto and performance by Rinde Eckert, and design and direction by Julian Crouch, the production combines music, theater, puppetry, instrument making, and scenic design.