What does Peter do that shows Annemarie that he is an adult now?

Present are an old man, a couple with a young baby, the Rosens, Peter Neilsen, ... at Ellen sitting between the Rosens and feels sad that they are in different worlds now. Ellen is headed for something Annemarie will not be able to share with her. ... Annemarie takes this as a sign that Peter has a place in the "world of adults.