MN Law for car seats.
I assume that Minnesota is pretty similar to other states when it comes to car seat laws so I figured I'd share what I know. First, there are 3 stages of car seats. Rear Facing, Front Facing, and Booster
A child must be in a rear facing car seat until they are 1 year old AND 20lbs. The AAP recommends waiting until a child is at least 2 years old to turn them front facing because of their immature skeletal structure. There is a push to change this law to at least 2 years old or they've grown past the max height/weight their car seat allows in the rear-facing position. Crash studies have shown that EVERYONE, even adults, would be safer in a crash if they were facing the back so the longer you can keep your kid rear facing the better.
Once a child has been transitioned to a front facing car seat they must use one until they are at least 4 years old AND 40lbs. Don't transition them just because they had their 4th birthday. The weight is important because of how a lighter body could move around during a crash. The age is important because of the maturing skeletal structure. Avery will be about 6 when she hits 40lbs according to her growth charts. Most front facing car seats will allow a child to stay in them until 60lbs!
If your child has outgrown their front facing car seat they must remain in a booster seat until their are at least 8 years old or 4'9" tall, whichever comes first. This is because most cars design their seat belts to fit an adult who is at least 4'9" tall. That's what they're crash tested for. If your 8 year old is very small it's best to keep them in some kind of booster seat until they're close to that 4'9" mark.
So lets review. Rear facing until 2years old. Front facing until 4yrs old AND 40lbs, Booster until 8yrs old or 4'9" tall. Not too confusing, is it?