Wait until your baby sits unassisted with steady neck control, usually around...

https://foxtrot-wiki.win/index.php/The_Safest_First_Ride_with_a_Baby_on_a_Bike:_A_Service_Writer%E2%80%99s_Guide

Wait until your baby sits unassisted with steady neck control, usually around age one. Front or rear seats suit toddlers best. Trailers accommodate infants using specialized inserts