Wait until your baby sits unassisted with steady head control, usually by age...

https://smoothdecorator.com/cold-weather-biking-what-should-your-baby-wear-in-a-bike-seat/

Wait until your baby sits unassisted with steady head control, usually by age one. Front mounts keep toddlers close for interaction. Rear seats offer stability as kids grow. Trailers provide the safest, protected space for long rides and naps.