I just use my eyes to look at them.
You're not looking at her feet. You're looking at a very low quality image of very loose-bottomed jeans and the toe part of a shoe. Without anything to show scale accurately. Some degree of accuracy is required, since just a couple of cm is a significant difference in the context of foot size.
On top of that, the degree of individual variation means that you can't tell a person's age from their shoe size anyway. And on top of that, a person's feet usually grow to their full size pretty early on in their life, before they're adults. Have a look online for average shoe sizes by age. You'll usually find the charts ending by about 12 years old.
Here, for example, is the most obvious by site name:
https://www.shoesize.com/youth-junior/sizechart/
Only goes up to 10 years old. After that it's adult sizes.