When you can see that she's hunting well enough and has good enough control of her limbs for you not to worry about her getting a mouthful. I'd guess around 3 months would be a good age. They're so puppyish and can be a little clumsy, but you could also just feed crickets in a separate container and keep her on soil from now on if you like. It's not as if she's likely to be too stressed or shy to hunt them in an unfamiliar container.