I'm just curious, because I, even though, DS is close to being 4, he is in no way (weight or maturity) ready for a booster seat. At a playdate yesterday, I was surprised to find out that one of the girls who is 4 years old, but my DS's size (30ish lbs) is already in a booster. In Illinois (where I live), there is no weight restriction on moving to a booster, so it's legal. I live near Missouri, whose laws are more strict, and I drive there with DS often. So, I'll be making sure I go by their laws at least. Anyway... I'm just curious how many people actually move their kids that early.
Marcey
Kaden William 11/4/06 and Dawson Michael 6/30/10
Dawson's first birthday - at the zoo

Re: How long do you plan to keep your DC in a 5 point harness?
Until they outgrow the seated height on the seats. Both of my kids are tall, so I will be trading in their current Evenflo Triumphs for Graco Nautili when they hit the top of the seat.
DS - December 2006
DD - December 2008
I moved Joey, at 4, out of his Marathon into a backless booster.
I will be moving Cam (soon) into a booster with back. She just turned four.
The minimum here is 4 and 40lbs but more importantly, they both outgrew their Marathons at 4. (I think Cam could probably stay in her Marathon a bit longer but I'm guessing she is near the top of the height requirements...so I'm not in a rush to get a new seat and will find out for sure at her appt next month).
Until they outgrow the height limit for their seats (they're skinny, so weight wont really be a factor). My DD will be 5 on Sunday and will almost certainly still fit into our Radian until she's 7 at least.
My DD was in preK last year and plenty of the kids in her school were in backless boosters at age 4. I think that's crazy and would never consider doing that, since there is a much safer alternative available.
My ILs have a booster with back in their van for DS. He rides in it OK most of the time but there are times when he's annoyed about being in the vehicle still and he'll put the seatbelt under his arm or even undo the buckle entirely.
I'll move him out of his 5 point harness once I am confident that he will sit in a booster properly 100% of the time - so I'm thinking maybe closer to 4.5-5 y/o.
DD is going to be 6 in September, and she is still riding in a 5-point harness. We only recently switched her out of a 5-point in DH's car and into a high-back booster because the crotch buckle was getting too snug for her. I plan on keeping her in her 5-point harness in my car (a Sunshine Kids Radian) until she's 6.5 OR when the baby needs it, whichever comes first. She's really comfortable in her 5-point, and actually isn't too thrilled with riding in/buckling herself into a booster at this point.
DD would not have been ready for a booster any earlier than this, though--despite being within the recommended age/size range, she just wasn't mature enough to sit up and sit still until recently.
i used to work in a peds office in IL... 4yr and 40lb was recommended for lap/shoulder belt boosters. here are the recs on the state website. https://www.cyberdriveillinois.com/departments/drivers/programs/kiss.html
scroll down and you'll see the description for each seat with ages and weights. not sure if this is law or recommendation... I know the "appropriate safety seat" is law until age 8
dc are on the smaller side. dd is narrow. I'll leave them in 5 pt for now. dd is 4 1/2, 40" and 32-33lb. ds is probably 36-37" and 29-30lb. both are still in their britax carseats (dd=decathelon, ds=boulevard)
I think DD is going to be in a harness seat until she gets to HS. j/k...probably
On the plus side, most of her friends (4-5 yo) are still in harness seats. I think it is going to start becoming an issue when she gets to school and we start joining carpools and such.
DD was five and we waited a few months until the bad roads season was over. Baby was on the way and we needed to put her in the way back of the van to make room for baby and I couldn't reach back there easily to get her in. She is in a booster with a back.
She is not yet 40 lbs, though. I felt badly about that and stressed to MIL that I'm not all that comfortable with her being without the 5 pt harness because that would be safer. Sure enough, though, MIL put the 5 pt harnesses away and put her in regular backed boosters in both of her vehicles because "you did it, so I can." Ugh. I'm not all the comfortable with MIL's driving in the first place. I never should have moved her out of 5 pt harness.
My kids will (or have gone) go into one as soon as they are big enough, old enough, and mature enough. I don't believe a 5 point harness benefits anyone who can use a booster correctly. Some disagree--it's kind of split, but there is absolutely no evidence that 5 point harnesses are safer than boosters when the booster is used appropriately. I know it won't be anywhere around 4 for Bella, but maybe 5? E went slightly after he turned 4, but he's big for his age, and very mature. Bella is lacking the maturity and the weight. I'm hoping to go straight from rear facing to boostering around 5.
It's simply not legal to have an under 3 or under 30 lb child in a booster in any state--the seat MUST be used according to manufacturer's suggestions, and no seat allows boostering under 3/30. I would never booster a child under 4/40.
It's like that here too. DD is in a booster. She is 5 (in 3 weeks) and 42 lbs. Her daycare requires them (booster seats only) for all kids 7 & under (afterschool program). We have prek 3 & 4 in public school here. DS is a skinny minny & short, he will not be moved as early.
Christian Alexander - 11/13/06
Amelia Rose & Owen Thomas - 3/29/11
Right now my thought is 6 years. I know DD will be close to outgrowing the Nautilus at that point. (She's 40 lbs and almost 44" now.) I'm still debating whether or not to buy the Frontier to keep her harnessed longer.
Most of her daycare classmates are in high back boosters already at 3 or 4 years old. DD has been asking when she'll get to ride in booster, and I've been telling her that she can when she's 6.
Charlotte Ella 07.16.10
Emmeline Grace 03.27.13
Until they outgrow the height on their Radians.