Most experts suggest waiting until your little one sits up unaided, usually...

https://jasperviex689.bearsfanteamshop.com/can-a-baby-sleep-in-a-bike-seat-a-shop-expert-s-guide-to-nap-safety-biking

Most experts suggest waiting until your little one sits up unaided, usually around nine to twelve months. Your baby needs solid neck strength to support a helmet safely. Test the helmet fit before your first ride to ensure it sits level on their forehead