Wait until your baby is 9–12 months old and sits independently with strong neck...

https://web-wiki.win/index.php/What_Age_Can_a_Toddler_Ride_in_a_Rear_Bike_Seat%3F_A_Mechanic%E2%80%99s_Guide_to_Family_Cycling

Wait until your baby is 9–12 months old and sits independently with strong neck control. Front seats allow for interaction, rear seats offer stability, and trailers handle rougher terrain best. Always prioritize a perfect helmet fit for every ride