Aww, what a cutie! In my opinion, I'd say she's probably about 5 - 6 weeks old, but I'm not an expert on rat ages ^_^
Since I read your post in Say Hello as well I thought I'd throw in some other stuff too. You should really get another rat as a friend to your little girl. Rats are very social and need to be with other rats. Also, if the aquarium in the second picture is her permanent cage, you'll need to get her into a good wire cage ASAP. Ammonia builds up quickly in aquariums because of the poor ventilation, and that irritates rats' sensitive respiratory systems. Also, I can't tell what kind of bedding that is, but it looks like pine. That also can cause respiratory problems because of the oils, so you should get her onto something like aspen or Carefresh Ultra ^_^
Read through lots of posts on here and ask lots of questions, because there is *so* much great information here. Most of what I know about rats came from here
