Wait until your baby sits independently with a steady head, usually by age one....

https://victor-wiki.win/index.php/Can_a_Baby_Ride_in_a_Bike_Seat_Without_Sitting_Up_Fully%3F

Wait until your baby sits independently with a steady head, usually by age one. Choose front mounts for interaction, rear seats for stability, or trailers for reclining safety. Always confirm neck strength first. Helmets are non-negotiable every ride!