Heather Corley, a certified Child Passenger Safety Technician, has answers about when you should move your toddler from a full car seat into a booster and what type of booster seat you should consider. Her recommendation:
"I suggest that parents keep their children in a harnessed car seat to at least 40 pounds and 4 years, but preferably longer. Today there are many car seats available that harness to 65 or even 80 pounds. Most children can fit into one of these bigger car seats well past age 4."
Your child needs to be able to sit properly for the booster to offer complete protection and for most toddlers, that's just not going to happen. It may seem easier for you to wrangle your child into the booster, but it's also easier for your child to move around, sit sideways and slouch there. Even when my son was technically ready for the booster seat, it was a struggle to get him to stop trying to sit sideways. The sides and harness prevented him from doing this in his car seat. So it can be worth it to wait before you make the switch.
Sometimes, too, you have to go above and beyond the law and do what's safest for your child. When it comes to booster seats, sometimes the laws haven't caught up with what studies and data tell us. When you finally make the move and you're thinking about when to move your child out of the booster seat, keep in mind that recent research shows that your child is safest in a booster up to the age of 8.

My daughter is 2.5 years old and still hasn’t even been switched FORWARD-FACING, never mind a booster. It makes me sick to the stomach to see such terrible car seat mistakes made by parents.
my 6 1/2 year old and my 4 year old are still in 5 point harness. they know – it’s the safest thing, and don’t even question it. convenience isn’t worth the life / health of your child. rather take the extra 2 minutes to strap them in.
A nice thought, however, sometimes you have no choice. My 2.5 year old is already over 40 pounds. He’s growing out of his 5 point harness seat, and pretty soon we’re going to have no choice. We tried a booster in my husbands’ truck and he’s not ready…too much freedom. I don’t know where you’re seeing 5 point harness seats up to 65 pounds, but I haven’t. It wouldn’t matter anyway, we don’t have the funds to buy 2 brand new car seats. My only option left is to stuff him in the 5 point harness I have until he just won’t fit anymore…which will be way before 4, but hopefully late enough.
@Becky: If your child moves around too much in the booster seat, try locking them in with the car seat lock – pull all of the webbing out on your belt, and it will lock them in place when you retract it.
I’m not sure what city this “booster” incident took place in but, it is hard when you live in a big city and depend on cabs vs. cars. We try to take public transportation as much as possible but sometimes we have to take a cab. Once our child moved out of the infant rear facing seat (that we could put on a snap-n-go) I was stumped. How do you lug a big convertable car seat around and push a stroller? We went with the Safety 1st Go Hybrid (http://www.safety1st.com/usa/eng/Products/Travel/Car-Seats/Booster-Car-Seats/Details/2384-22256AHE-Go-Hybrid-Booster-Car-Seat). It installs in less than a minute, he’s in a 5 point harness and it folds into a bag that I can hang on the back of his stroller. The seat should be sold as a “city seat” so that more city dwellers are aware of the option. I hope the person using the booster was hopping in a cab and just wasn’t aware of this option.
Both Britax Marathon and Boulevard have 5 point harnesses and go up to 65 lbs.
We have a safety 1st that is a 5-point harness up to 65 pounds and is significantly cheaper in price not quality than britax. You can find them online.
to all the mothers… it is not an easy task, and I am sure God takes care of all of our children in the situations when we can’t. But, the point is not to try to convince ourselves that the least we can do is the best we can do. Britax frontier has harness up to 85lbs iI believe and the converts to a booster. About having or not the means to get something better, well do not ter yourself appart about being a good parent or not… But it is not acceptable for a 2yo to be on a booster. it is even illegal. so try going to thrift stores, salvation army, garage sales, fire house department of transportation… there are programs, explain your situation. that is the least you can do. besides the car seat is not only so the child doesn’t fly of the car, it is used to absorb the impact before it reaches the body of your child. so there is always a way. under stress, we don’t think clearly, you are all better parents if we look for answers and push extra even if it doesn’t seem possible. good luck to you all
It all depends on the child, I have a very active boy but he was in a booster at about 3 1/2. He was 40 lbs and hadn’t outgrown the 5 point harness, but he was undoing the top clip and taking his arms out of the harness and I tried everything from putting on “child proof” piece that clipped over it to a more complicated clip and nothing worked. Out of sheer frustration I put him in a booster and doesn’t complain. He sits upright and doesn’t move. He just wanted the freedom to move forward to grab something if he needed to.