Hi willowgram! 6"... likely you have a female but she still has some growing to do! Can you post pictures of her shell? Plastron and carapace? If her markings are still vibrant and she still has some growing to do, I would say 3-4 years old. She could be younger if she was overfed and has grown quickly to her current size.
As for bonding, their personalities vary. But once she associates you with food, you'll be her best friend