B.H.
My daughter is 6 and in first grade she's in a Britax Regent a 5pt. harness carseat. It goes up to 80lbs.
I tried a booster thinking they're cool and my kid would be a big kid. They just don't work for us. She falls asleep and slumps over and looks ridiculous. I've tried them as a spare seat at 3years,4years,5years, and now 6years old and at 6 she is finally fitting better and looking safe. I still opt for our 5pt. harness though.
I would keep your child in a 5pt. harnessed carseat as long as possible. I'm not trying to sound like a know it all stuck up parent or anything. I just know that car crashes are the #1 reason for death in children.
Becareful when shopping for carseats. Most carseats out there go to 40lbs. But some carseat/booster combo's claim 80 or 100lbs.. But that's not with a harness that's with seatbelt aka booster style. Those seats donot work good for boosters, they work better for the harness part up to 40lbs.
If you must get a booster Graco works the best in my experiance and opinion. Graco Turbo Booster
MN law was 4year or 40lbs. but..,, I believe that is about to change if it hasn't already to 8years/ or 80lbs. As long as your child is in a childseat of some sorts you won't get a ticket but it's not like they have the time to make sure your kid is in the appropriate seat for age or weight.
Generally it goes like this:
Rearface atleast until 1yr.-20lbs. but now they're recommending to extend that til 35lbs.
Typical toddler 5pt.harness carseat til 40lbs.
Some ppl choose to buy carseats that harness til 65 or 80lbs. though
With booster's it's
30lbs and up for highback booster
40lbs and up for the short style boosters
they also have height requirements and I don't know those off the bat.
Check out this website. It made me cry!
http://www.youtube.com/watch?v=azgBhZfcqaQ