It's not so much the age, it's the size and maturity of the child. I believe the Britax Frontier85 currently has the highest top slot height of all the HWH (high weight harness) seats on the US market, with a weight limit of 85 lbs in harness mode. Most kids will outgrow the harness far before they reach 85 lbs, however. The top slots, if I remember correctly, are 20 inches from the seat bottom. You may lose a little room on that depending on the recline after it's installed. Your child has outgrown the harness mode when their shoulders are level with the top slot (not below), so it all depends on your child's size and build (torso length being the most important measurement). My DD is 6 1/2 years old. She's around 47 inches tall and 55 lbs. She's still harnessed in her FR85, on the top slot, with a little growing room left. She has a long torso. She's only harnessed because she isn't quite mature enough to stay positioned properly in a highback booster - once she gets to that point, hopefully by spring, she will be moved to booster mode. A booster is a perfectly safe option for a child who is compliant and fits well in a booster. I, personally, wouldn't want to re-harness an 8-year old, but if it's her choice and she's more comfortable, I guess its something to consider. At 8, there likely won't be much growing room left, if any, in something like a Frontier85 (but it does convert to a highback booster as well).