My girl is 6.5 and still harnessed in a Regent in our main vehicle, she is just about to move up to top slots, she is not tall by any means, I think she could stay harnessed forever to be honest.
The reason she is still harnessed is that she LOVES it and her Regent, and dislikes riding in the booster seat, the Regent still has 3 years left on it, I don't have another use for it, and she states she is never coming out of her comfy pink seat.
BUT, she does ride in a Turbo in dad's car now, which is about 4 times a week school pick up, 20 minutes. She does fine, not as comfy in it, and complains time to time but the belt fit is awesome on her, and I feel it's getting her used to it, I also use it when she goes with her best friend in their van.
I know she will stay in the Regent through the summer, because of traveling, so she can rest and stay in position better on the long trip. I will probably use the booster in the third row of my van when we have Grandma here for 2 weeks in late summer, so she can ride in the captain's chair. Not sure if I will re-install the Regent then for her, or go ahead and go to booster full time, my guess would be booster full time, she will turn 7 in August and enter 1st grade. I will still use the Turbo in dad's car and will be getting another full time booster in the van that has more side protection and overall comfort for her on a daily basis.
She is currently 44 lbs and 45 in.