Art is a reflection of life. Life and surroundings are observed for painting and pictures. Books, movies, and music often reflect current events, trends, and styles of the day. Most artists take what is going on around them and create from it, or are inspired by it. In this way, art imitates life.
I do not believe art solely imitates life or that life solely imitates art. Being an artists means you can pull things either way. Therefore, you may create something based off of your life but at the same time people may imitate that in their life. It's like cause and effect, you can't really figure out what came first.
In the beginning, art was just depictions of daily life. People created art that reflected how they lived, what they worshiped, and who they were. Soon, these features were used as references for how life should be. "We harvest at this time because the wall drawing says that is the best time." Life has not changed much beyond that. We create art to reflect the things we see happening or the things we expect to happen. If life imitates art, It's 2015, and according to the movie, we should be getting hoverboards this year.
The old maxim and the old saying is clever and profound, but obviously art imitates life. Life comes first - both before the concept of art and before an artist today actually understands what art is. In some cases the opposite is true, but overall art imitates life in the world.
Yes, art imitates life because it is life that defines art. Art takes characteristics given to it by life to imitate emotions, feelings, and ideas. So in this aspect, art is viewed and created through life. When an artist paints a picture, he is putting his own life into it through his emotions, imagination, or experiences. The art is taking on and imitating what life has to create it.