Wait until your baby sits upright with strong neck control, usually around 12...

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

Wait until your baby sits upright with strong neck control, usually around 12 months. Front seats keep them close for chatting, rear seats fit growing toddlers, and trailers offer the safest, stable nap space. Always clear it with your pediatrician first!