Most little riders join the fun around 12 months. Look for strong neck muscles...

https://mike-wiki.win/index.php/Are_Bumpy_Roads_Dangerous_for_Babies_in_Bike_Seats%3F_A_Service_Writer%E2%80%99s_Guide_to_Safe_Cruising

Most little riders join the fun around 12 months. Look for strong neck muscles and the ability to sit up independently while wearing a helmet. Trailers offer great stability for beginners, while front or rear seats keep them close