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

https://tiny-wiki.win/index.php/My_Child_Slumps_to_One_Side_in_the_Bike_Seat:_Is_That_Normal%3F

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