Wait until your baby sits steadily with strong neck control, usually around 9...

https://www.bookmarkingtraffic.win/most-toddlers-are-ready-for-a-bike-seat-once-they-possess-strong-neck-control

Wait until your baby sits steadily with strong neck control, usually around 9 to 12 months. Choose a front mount for interaction, a rear seat for stability, or a trailer for cozy naps. Always fit their helmet perfectly before every ride.