Re: Are there any dangers to harnessing a child past a certain age?
We don't know which is safer. It may be that in some crashes, an older kid is safer being boostered, and in other crashes, that same kid is safer in a harness. Or it could be that one or the other has an advantage across the board. We just don't know.
We *do* know that in Sweden, which has a history of emphasizing child passenger safety, kids typically move to a booster seat around age 5.
In your case, since a properly used harness IS safer than an improperly used booster, then a harness might make more sense (if you are pretty sure a booster would not be used properly).