The five step test is a minimum to get out of a booster.
It would be a parental decision to use a booster beyond the stated height limit. This made me realize that I didn't know the stated height limit for my 11.5 year old's (86 pounds, 57"/58") booster. It's 57". The belt fit is still good and she's within the weight range, so I'm going to make the parental decision based on talking with several companies about their overall height limits to continue to use the booster until she fits better in the car and has a more mature skeleton. Also, she doesn't want to get out of it, she says it's very comfortable.
So it's up to you. You can look into the Safety 1st Incognito with it's 60" overall height limit and $20 price tag if you'd like to get them something else that they are within the range to get them another year or two until their skeletons are further along. My daughter has one in my husband's car (and she'll be using it to over the limit as well I bet. I don't pass the five step test back there at 5'1", the back seats are too deep). I've also talked to Safety 1st about using it over the height and would have no issues whatsoever going above the stated 60" limit.
Wendy