user: I'm considering showing the movie "Avatar" at my family's movie night this weekend, but I need to make sure it's appropriate for all ages. Can you find out the age rating for "Avatar" for me?

Available functions:
name: get_movie_rating
description: Fetches the age rating of a movie from the OMDB API.
parameters: type: dict
properties: movie_name: type: string
description: The name of the movie.
required: movie_name