Most babies are ready for a bike seat once they sit upright unsupported,...

https://www.pfdbookmark.win/wait-until-your-child-sits-up-and-supports-their-own-head-usually-near-age

Most babies are ready for a bike seat once they sit upright unsupported, typically around nine to twelve months. Look for strong neck control and ensure their helmet fits securely. For younger babies, a trailer offers the best reclined stability