Well, that's a complicated question. I have a timeline in my head of how old she should be and when. It really depends on when she meets Rex. In AAA, she's in her early teens, and she joins the Resistance soon thereafter. If you want Rex to meet her while she's a Resistance agent, I'd go with 15. If you want her to still be part of the Empire (in the Academy Automicus, I would say 13-14. A few years later in the timeline is when she started her shop.
Maybe Rex meets her when she's running errands or smashing automatons. She also likes climbing trees (one of her battle tactics is to drop down on an enemy from above). Whatever works best for your story.