Bailee’s at that age right now where she could be passed off for someone a year or so younger than she actually is, or she could be made up to look a few years older than she actually is. She’s 14, but if you see pics of her at official events where she’s dressed up, she looks late teens.
Plus, there is that gap of time between Kid Snow and Adult Snow. They can easily have her do flashbacks that take place within that span of time, while making sense of the timeline with Bailee’s aging. I believe Adam and Eddy hinted at maybe doing more flashbacks of Snow when she lived with Regina. So theoretically Bailee could do a flashback that takes place 2 years after the events of “The Stable Boy.”