I wouldn't use the third one.
There could be a slight difference between them, depending on the context.
Let's say you and I are talking, and you have a daughter named Jane who is now twelve.
You tel me that you met a lady yesterday who also had a daughter. You can say (about the daughter) either "She was Jane's age" or "She was the same age as Jane."
Now, different conversation. You tell me that Jane wants to visit your parents, but you are worried she's too young to fly alone. "Don't worry," I tell you. My daughter flew alone two year ago and she was Jane's age." -- That means that she was 12 THEN, even though my daughter and your daughter are not the same age.
Does that make sense?