It depends on what game you're talking about. In Final Fantasy VII, she was 16. In Advent Children, she was 18. In Crisis Core, she was nine, and in Dirge of Cerberus, she was 19. I hope that answers your quesiton.
Once the battle is over, your party and Yuffie will appear on a small map with a save point. Yuffie will be lying down, tired from battle. DO NOT USE THE SAVE POINT. Yuffie is a materia thief. You...