4 years old
Almost four is NOT four.
And truth be told, four is too young for a booster. Why? Because a child in a booster seat needs to be able to control their impulses. They need to be able to sit still. They need to be able to not fall asleep. They have to be old enough and mature enough to resist the urge to lean forward to get a toy, or especially to unbuckle their seat belt to get to something they can't reach otherwise. They need to be able to NOT do all the things we do as adults. Generally speaking, the minimum age that kids can do this is 6, sometimes 5. Rarely four, and never three!
Reasons you may want to use a booster
I know it's convenient to use a booster seat. There's no harness adjustment, the kid can buckle themselves in. Getting in and out of the car is so much faster! Trust me, I have to harness three kids under four (read: squirmy / sticky kids) in car seats every time I go somewhere. I have to get them out of their harnesses when we get where we're going, and I have to put them back in when we're done. This does in fact make quick errands a total pain in the butt. I either suck it up or make my husband run those errands sans children (or make him watch the children while I do it myself). Like all three year olds, mine is willful, defiant, and has an over-developed sense of independance. Honestly, this just means he has all the more reason to stay in a harness!
And of course a booster seat a milestone that means out babies are growing up. *bitter sweet tear* But just like every other growing up milestone, reaching it too early isn't a good thing. Would you make your 6 month old learn to walk, or feed your 2 week old baby strained peas? No, probably not, because you know they aren't ready for that kind of development. Likewise, your 3-4 year old kid isn't ready for the kind of development it takes to sit safely in a booster seat, and you shouldn't make them.
Reasons you should use a booster
The ONLY reason to use a booster seat is that your child has outgrown his five-point harness by either height or weight. Since the average, cheapest, forward facing car seat harnesses to 40 or 50 pounds, I find it hard to believe that an average three-year-old has outgrown it. Some three-year-olds, yes - of course. Every child is different. But if that's the case, then the correct decision is to buy an expensive car seat. I know it's tight on a lot of budgets, but being unsafe isn't a valid option. Sorry, but it's not, and boostering a child that's too young for it is unsafe.
Examples of Booster Positioning
This is MY 3-year-old. He is 43in tall, and he weighs 63 pounds. I am hard pressed to find another 3-year-old that is as tall or as heavy as mine. I'm sure they exist, I've just never met one. Here is Luke in a booster seat.
The shoulder strap should be across the middle of his shoulder - in this picture it's too close to his neck.
The lap belt should be across his thighs, but in this picture it's across his soft belly, which encases his soft internal organs. Do you think that the seat belt forces during a collision should hit him in the stomach?
This is NOT a good Booster seat fit at all.Point I'm making: ff my enormous 3-year-old doesn't fit in a booster seat yet, then your average sized 3-year-old most definitely doesn't. And proper fit is only the first requirement.
This is what a correct fit looks like:
Side by side for an easy comparison:
High back booster good; No/low back booster bad.
Again, not always bad, but bad if the child isn't ready. A high back booster (like both pictures above) provides protection in the case of a side collision. Instead of an outbound seated child's head hitting the window of the car, his head will hit the nicely padded, soft cushion that is part of his booster seat. And if your child is belted in the middle position, then his head, neck, and spine have a much shorter distance to travel to one side before being stopped. Why would you ever NOT give them that as long as possible? The only reason to use a low back booster is if the child has outgrown the back.
Reasons to stop using a booster seat
"I'm going to be laughed at" isn't sufficient. While it is heartbreaking to think of our children being teased, and while instinct may tell us to prevent it at most costs, this is not one of the costs to risk. Giving up the booster seat too early could mean the life of your child - they still need to be protected even if it's potentially embarrassing. The only valid reason to take your child out of a booster seat is if the safety belt fits them better without one.
Don't believe be yet? Want some crash test videos? Here you go!
Side impact with a high back booster:
Backless versus high back:
Submarining: The motion of an occupant when one or both hips slide under the lap belt, so that the belt applies crash forces to the soft abdominal area between the pelvis and ribs
Last but not least, here's an ad from Britax for their booster seat. All marketing aside, they have some informative drawings detailing the issues with booster seats if not used correctly. And if your child is too small or not mature enough, the seat won't be used correctly.
Had enough? Here's one more. Meet Kyle. He was in a booster seat, but the buckle on his seat belt failed. Had he been harnessed and installed with the seat belt, it still would have failed, but he would have been tethered with the top anchor. The seat would not have been ejected. And if he wasn't tethered, the seat would have been ejected WITH him, as opposed to just Kyle being ejected. It would have still provided a bit of protection, and while he would have been seriously injured, he may have survived.
There are few decisions we can make as parents that come with guarantees of safety or happiness, but we still do the best we can. Don't let a booster seat be a bad decision.