I think, in the snake world, age is not a great way to determine size. She looks very healthy to me. If she shows signs of being hungry a couple days after being fed, I would bump up feeder size, but otherwise if she is calm between feedings, she is fine. With my 7 y/o Blood, I had her on medium rats when she was over 7 lbs, and gave her a large rat every other feeding. When she started getting restless on those "medium weeks", only then did I permanently bump her up to large rats. I was so cautious because she went on a fast for a few months, years ago (which, from what I've heard is pretty unusual for Bloods) after I offered a larger size than she was used to. Your girl has lots of time to grow.