Most experts suggest waiting until your little one sits up unaided, usually...
https://www.red-bookmarks.win/most-toddlers-are-ready-around-age-one-look-for-solid-neck-strength-and-the
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