“It took me a while to get into screenwriting and filmmaking. I started out as a playwright, and I’m still a playwright, but I was in my early thirties before I ever tried to write a screenplay for myself. The theater taught me to respect actors, the advantages of rehearsal, the magical power of theatricality, the plain fun of putting on a show, and the unique thrill of creative collaboration.”
Oscar-winning screenwriter Kenneth Lonergan (Manchester by the Sea)
Writers Guild of America, East interview
P.S. I should add that according to Wikipedia the Lonergan began writing in high school when he went to a private school in Manhatten that had an arts program, had a play selected for the Young Playwright’s Festival when he was 20, and graduated from the playwriting program at NYU. He had several plays produced in New York and worked as a speechwriter for the Environmental Protection Agency before his success as a screenwriter.