Wait until your child sits independently and holds their head up well, usually...

https://sticky-wiki.win/index.php/How_Long_Can_a_Baby_Stay_in_a_Bike_Seat%3F_A_Service_Writer%E2%80%99s_Guide_to_Safe_and_Comfortable_Cycling

Wait until your child sits independently and holds their head up well, usually by age one. Choose front seats for interaction, rear seats for stability, or trailers for cozy naps on longer rides