Read the excerpt from "I'm Not Thirteen Yet" by Amy Bernstein.

Lynette started bringing new thirteen-year-old friends to our lunch table. Then Lynette and these new girls began holding thirteen-year-old clubs after school. Obviously they were far more mature with the word "teen" attached to their age. It wouldn't be long before there was tape down the middle of the cafeteria, dividing it in two. If you weren't thirteen, you dared not enter the thirteen-year-old side. If you tried, the lip-gloss army would hurl you back.

Gradually, Lynette stopped inviting me over after school. One day, on my way into the girls' bathroom, I overheard Lynette and Shelley Abrams discussing their after-school plans at Dale's Ice Cream.

The narrator is excluded from the thirteen-year-old clubs because

1.she is not liked by anyone.
2.she is not old enough yet.
3.she is too shy to join.
4.she is too immature.

In the excerpt presented, the narrator explains her friend Lynette changed her behavior and began to be with only thirteen-year-olds, leaving her alone. This is explained in "Lynette started bringing new thirteen-year-old friends" and "Lynette stopped inviting me over after school".

Besides this, the narrator explains "If you weren't thirteen, you dared not enter the thirteen-year-old side", these words suggest the narrator is younger than Lynette and her friends, and therefore this is the reason she is being suddenly excluded as she cannot belong to the groups and clubs were Lynette is. According to this, the narrator is excluded because she is not old enough yet.