This study aims to explore how well **Teplizumab**, a medicine given through a drip (IV infusion), works for young people with **Stage 3 Type 1 Diabetes**. Type 1 Diabetes is when the body can't make insulin, a hormone that helps manage blood sugar. The study compares Teplizumab to a **placebo**, which looks like real medicine but isn't, to see if it can help control blood sugar and reduce the need for insulin over a year. Participants will be between 1 and 25 years old. Key Points: - The study lasts about 18 months (84 weeks) for each participant. - Participants must be diagnosed with Stage 3 Type 1 Diabetes and be able to start the study within 8 weeks of diagnosis. - Participants will receive either Teplizumab or a placebo through a drip. Before joining, participants will be checked to ensure they are suitable, including tests for diabetes and other health conditions. It is important for potential participants and their families to review these details and discuss any concerns with their doctor.