Is it safe for a 9-month-old to wear a necklace?
Any object around the neck of any infant
or young child has the potential for injury. Such situations are rare, but they can occur. If the necklace gets caught on another object, it could be pulled tightly around the child's neck and cause problems. I can appreciate the value of such a treasured item for your daughter given by her grandparents. Certainly you could put it on her for brief periods when you are holding her, to have her photograph taken for example, or during her baptism or christening ceremony. But don't allow her to cruise around on her own wearing it.
I have not seen any specific recommendation for ages when a necklace is considered safe. During active playtime, even for older children, any jewelry could get caught or entrapped in a piece of equipment, a clothing tag, or another child, and possibly cause harm. Allow your daughter to wear her jewelry when you can closely supervise.