My son won't turn five until the summer, but we plan to keep him harnessed until he reaches the limits of his Regent and Nautilus. Unfortunately, that time is not too far off, as he has a long torso, and is on the top slots on both seats, with very little room for growth in the Nautilus especially. He is about 45 inches and 55 pounds. The Nautilus will have to go to booster mode within a few months, but I hope we can get at least another year from the Regent.
I was aggravated yesterday to see one of our young relatives already in booster mode on the day of his 4th birthday. The dad said that he had been in the booster for a few months already because he "did not fit comfortably" in any harnessed seats anymore.
The little boy is just about 40 inches and 40 pounds; bare minimum for a booster.