Most experts recommend waiting until your baby reaches the 9 to 12-month mark...

https://wiki-legion.win/index.php/What_is_Safer_on_a_Busy_Street:_Trailer_or_Rear_Seat%3F

Most experts recommend waiting until your baby reaches the 9 to 12-month mark before riding. Your little one needs rock-solid neck and core strength to hold their head up comfortably while wearing a helmet