Pre-teen Holmes

According to Deadline, Stranger Things actress Millie Bobby Brown has signed on to star in a movie adaptation of the popular Enola Holmes mystery series, about the crime-solving baby sister of Sherlock and Mycroft Holmes. She's also apparently producing the movie, which I'm finding a little confusing. Isn't she, like, 13? Why does producing a movie entail? I had assumed it involved more than middle-school level skills, but apparently not.