Wait until your baby sits independently and holds their head strong, usually...

https://www./parenting/when-can-a-baby-go-in-a-bike-seat-an-age-by-age-guide/

Wait until your baby sits independently and holds their head strong, usually around age one. Start with a stable trailer for younger infants, then graduate to front or rear seats as they grow. Always fit a helmet correctly and ride safely together.