Act I is "the Challenge." For the first third of our life, we would be discovering who we are. We would be introducing ourselves to the world. And in the end of the first Act, we would accept a challenge. We would decide about our purpose in life... whether as mother, or in business, or as an artist. Who knows? We would decide what we were striving toward.
Act II is "the Action." During middle age, we would develop as people. We would come across many troubles and experience the darkest moments of our lives.
Act III is "the Consequences." The climax of our lives would occur: the defining moment. Life would mean something.
Maybe life does have a plot...