I would say it depends on the baby. My first grand daughter was a year old and was barely in 4's. Then I have Sage my youngest grand daughter who is not quite three yet and she wears an 8-9 and her six year old sister wears an 11! These shoes belonged to Sage which is why they were only worn a couple times. I would say a good guess would be between 3-9 months? Not sure though. Maybe someone else could comment and help out there...
I want to add that in the very first picture the shoes appear dark around the silver part but this is just the way they came out in the picture from the camera. They are clean and barely used as you can see in the other pictures.