Wait until your baby sits unassisted and holds their head steady, usually...

https://gunnerawsh736.cavandoragh.org/why-does-my-child-bike-seat-squeak-and-rattle-the-whole-ride

Wait until your baby sits unassisted and holds their head steady, usually around nine to twelve months. Always prioritize neck strength over age. Before your first trip, ensure a toddler helmet fits snugly