Wait until your baby is 9–12 months old and sits up unsupported with strong...

https://cashwpkr081.huicopper.com/what-age-can-a-toddler-ride-in-a-rear-bike-seat-a-mechanic-s-guide-to-family-cycling

Wait until your baby is 9–12 months old and sits up unsupported with strong neck control. Use front seats for bonding, rear mounts for stability, or trailers for longer, nap-friendly hauls. Always start slow and prioritize a secure helmet.