This study looks at how safe and effective a new vaccine called **PCV21** is for preventing infections in children. PCV21 is compared to another vaccine called **20vPCV** (Prevnar 20) in kids who have not had all their vaccinations on time. It includes babies as young as 7 months up to teenagers 17 years old. **Key Points:** - **Study Length:** Babies will be in the study for 9 to 11 months, toddlers for 8 to 9 months, and older kids/teens for 6 months. - **Eligibility:** Participants must be healthy. For older girls, they must not be pregnant, and they should agree to use birth control if necessary. - **Exclusions:** Children with certain medical conditions, past specific vaccinations, or who have received other vaccines recently cannot join. Participants and their parents must attend all visits. This study ensures the vaccine is safe and works well for different age groups. Always talk to your doctor to see if joining this study is right for you.