Maddie Ziegler, Maisy Stella, and Kerrice Brooks have brought their coming-of-age dramedy “My Old Ass” to Sundance. They and their director, Megan Park, stopped by the IndieWire Studio presented by Dropbox for an interview about the film, which sees Stella’s character suddenly able to interact with an older version of herself (Aubrey Plaza), who has plenty of life advice to share.