If she's old enough to be out on the street alone, then she needs to know the risks and how to stay safe. She doesn't need to know graphic details yet, and you don't need to communicate an attitude of fear, just that people who comment on her body or who are too interested in her are not safe to be around.
I was maybe 8 or 9 hanging out in the road in front of my house with a couple other neighborhood girls when a guy in a car started repeatedly driving back and forth and slowing down. Maybe he was looking for an address or maybe he was creeping on us, but we knew not to stick around and find out, so we vacated ASAP.
I knew by that age to ignore catcalls and at all costs to avoid men who catcalled me or commented on my body. I didn't know exactly why until I was a little older but I knew it wasn't safe and I knew there was physical danger to myself involved.