Babies generally reach bike-seat readiness between nine and twelve months....

https://sticky-wiki.win/index.php/How_Long_Can_a_Baby_Stay_in_a_Bike_Seat%3F_A_Service_Writer%E2%80%99s_Guide_to_Safe_and_Comfortable_Cycling

Babies generally reach bike-seat readiness between nine and twelve months. Check that your child sits up independently and holds their head steady while wearing a helmet