Wait until your child has strong neck control and sits independently, usually...

https://research-wiki.win/index.php/What_is_the_Safest_First_Ride_with_a_Baby_on_a_Bike%3F

Wait until your child has strong neck control and sits independently, usually around age one. Use a trailer for younger ones. Once they have core strength, pick a front seat for an interactive view or a rear seat for a stable, comfortable ride