Another good indication of age is the ear. How much of the ear has unfolded. Between that and the eye you can guess pretty well of their age. I don't remember exactly the amount of unfurled ear to weeks in age.

If it managed to jump to your window sill it may be older than you think.

Do give her high protein food. We mix kitten "milk" formula with wet food for kitten then gradually decrease the kitten "milk" formula gradually. Milk formula needs to be at the consistency per instruction on the container.

At 5-6weeks they should be mostly on wet kitten food.