They can start at any age, given you can find something that would fit.
I had to make my own leese (what we call leashes -- long story) because I didn't like what was sold in the stores.
Given yours is still a baby, you could simply use a shoelace (sneaker types are best) and criss-cross it across her chest and loop it on her back to create a "hook". As she grows, you could experiment with ones sold in stores, perhaps for ferrets or other small animals, and see which ones work best for her.