A 15-year-old boy is newly diagnosed with type 1 diabetes mellitus (T1DM). The nurse practitioner is educating both he and his mother about the disease. Which of the following statements is incorrect?

"This disease may be linked to autoimmunity."

"The age of onset is usually prior to age 20."

"In this disease, the beta islet cells of the pancreas are destroyed."

"Metformin is the first-line medication for treatment."