It's such an individual thing, there is no across-the-board answer. Too many factors involved.
My oldest DS moved into a highback booster fulltime at 5 years and a few months old. He was around 48-50 lbs (he had been in a Marathon). He had no underlying issues and he was mature enough at that age to sit properly the entire length of every car ride. He was also mature enough to know that he was responsible for buckling in his booster each time he got out of it. He wanted to be in a highback booster and I didn't really feel it was necessary to keep him harnessed any longer, given his maturity and understanding of what HIS resposibilities were.
My DD, on the other hand, is still harnessed now, at 7 years old. She will most likely be harnessed until she outgrows her Frontier 85, which will be in about 3/4 of an inch. She *does* have some underlying issues and does *not* yet have the maturity to sit and stay positioned for an entire car ride. She also still prefers to have a harness.
I don't know which way my youngest will go at this point, but if he's mature enough to stay positioned properly by the time he's inching up on his 6th birthday, I have no problem with him being in a booster. I'd like him to stay harnessed through age 5, I think. But 6 in a booster, provided we have no issues, I'm fine with.