H has used a booster part-time since not quite 4, about 43-44 inches tall and 42-44 pounds (she hit 40 pounds shortly after turning 3). At first it was VERY part-time, only a few minutes at a time in her dad's truck (she used a Husky in my vehicle), a few blocks up the street to the store and back home again. Shortly after she turned 4 I got a new (to me) vehicle that was incompatible with the Husky, so we made the plunge, since she was so big, to the Parkway. Sleeping was absolutely fine in the Parkway, no head slumping whatsoever, even without locking shoulderbelts,
but she was definitely not ready to use a booster for longer trips. She would forget to sit straight, she would lean way out of her booster to look out the rear window, she would lean way out to the side or down in front of her to reach something.
I always had to remind her to sit back. Thankfully Julie and some other wonderful folks helped me out, and I acquired a Fisher Price Futura (discontinued, harnessed to 60 pounds) and a Britax Laptop (discontinued, foam restraint to 65 pounds) so we could relegate the Parkway to spare seat status. She fit the Futura for a year and a half, and I was ecstatic!
During that time she gradually used her Parkway for longer and longer trips, and I could see her impulse control was getting better. Her buckling skills got better, too. She used to have tantrums when she couldn't get the seatbelt buckled quickly and easily without twisting the belt, I think because buckling her harness was so easy for her. When she was on the verge of outgrowing the Futura I acquired Darren's Husky (because I bought a new vehicle and it was compatible, woohoo!) and that became her primary seat again for awhile. A few months ago I decided to let her use her booster on a long trip and she did beautifully.
She's used the boosters ever since, and I've only had to ask her to sit back one or two times. She's doing really great.
So I concur with the others, some kids can handle boosters at age 4, but I would say that most cannot handle it full-time until more like age 5-6. H has some behavior/hyperactivity/impulse control issues anyway, so I fully expected to need to keep her harnessed until age 7-8, when she presumably would have outgrown the Husky, but with all the CPS education she gets from me
and all the practice runs in the booster in her dad's truck, it all finally clicked for her and she handles it very well now.
In addition to the maturity issue, though, there's the study that said kids under age 5 have higher rates of head injury if they are riding in boosters than kids over age 5 (presumably because younger kids have heavier/larger heads in proportion to their bodies, so they fly forward farther or with greater force, thus incurring greater injury if/when they come in contact with vehicle interior or intrusion?).
