Start biking once your baby sits well unsupported and holds their head steady...

https://xeon-wiki.win/index.php/When_Should_I_Switch_From_a_Bike_Seat_to_a_Trailer%3F

Start biking once your baby sits well unsupported and holds their head steady in a helmet, usually around age one. Choose a front seat for bonding, a rear seat for growth, or a trailer for extra stability. Keep your first rides short and fun.