Most infants need solid neck control to ride safely, typically around nine to...

https://alpha-wiki.win/index.php/Is_a_Suspension_Seatpost_Worth_It_for_Riding_with_a_Baby%3F

Most infants need solid neck control to ride safely, typically around nine to twelve months. Your child must sit up independently without support before hitting the road. For the youngest riders, trailers offer the most stability and comfort