DS1 passed the 5 step test in DH's car and our carpool's car (sedans) when he was 10 years old. He didn't pass in my car till he was a few months past 11.
My carpool boy is 11 years old and in 5th grade, but is only about 2" taller than my 6 year old DS2. He's pretty much the size of a first grader and is no where near passing the 5 step test. He stopped using a booster in his own car this year, but I still make him sit in a backless booster in my car. I think he actually prefers sitting in a booster in my car because he can actually see out the window and the seat belt is not hitting him in the face. I drive him every single day to or from school on the freeway at 70 mph and wouldn't be able to live with myself if we got in a crash and he was injured because he wasn't in a booster when I knew he needed one. I told his mom that and she was fine with me making him ride in a booster in my car. She can do what she wants when she drives him. There's now the $20 Safety First Incognito booster that is so low profile and matches the car interior that it's pretty much barely noticeable. DS1 had to use it in my Mazda5 captain's chair for the first few months of middle school (6th grade) since he didn't pass the 5 step test yet. He didn't mind since no one could even notice it since it's like 2 inches off the seat! Car accidents are the number one cause of death in children of all ages and if a seat belt doesn't fit without a booster then it's not going to protect you properly in a crash. It’s important to understand that a child’s body is not just a smaller version of an adult body. There are many physical changes in bone density and bone formation that happen just before and during puberty. The best advice is to use a booster if it’s needed up to age 12 years when some of those internal changes to bone density and structure (especially in the hip bones) have had a chance to mature. Which is why a small adult, who's bones are mature, will probably fare better in a crash without a booster than a pre-pubescent child without a booster. Isn't this correct Jools? One of the girls in DS1's 5th grade class last year (so 10 or 11 yrs old) was in a car accident with her mom and 2nd grade brother last year. Both the mom and the DD (who was not in a booster) were in the hospital for several days with several broken bones and injuries. The DD was in a wheelchair and then crutches for months. The little brother, who was in a booster, was perfectly fine with no injuries at all. I really don't see why people are so against using boosters when there's so many options nowadays.
Safety First Incognito
http://www.amazon.com/Safety-1st-Inc.../dp/B00FLXBHIU