First off, know the weight limits on your car seat.
For Brentley's infant carrier, the weight limit was 5 to 22 lbs. Since my little man is a chunk, he outgrew that at around 5 months old. Grace and Brentley have the same convertible car seat now. The weight limits on it are: rear-facing up to 40 lbs, forward-facing up to 60 lbs, and high-back booster up to 100 lbs.
There are weight limits on a car seat for a reason. If the weight limit on your baby's infant carrier is 22 lbs, do not place your 25 lb baby in it.
I am all for extended rear-facing, but if the rear-facing limit on your child's car seat is 40 lbs, then you cannot continue to rear-face your 45 lbs child. In the event of a crash, the seat would not function properly if the child was over the weight limit.
Most, if not all, car seats have the weight limit listed somewhere on the car seat itself. And it will also be listed in the car seat's manual. If you haven't noticed by now, that is one instruction booklet you don't want to throw away.
Now, let's move onto weight limits over-all. Most states have a law that requires children under the age of one year old AND 20 lbs to be rear-facing. THIS IS A LAW in most states. Not a choice.
In Georgia, a child who is under 4'9 and 40 lbs OR under the age of eight MUST be in a car seat per the law. If your child reaches 4'9 and 40 lbs before the age of eight, then it is legal for them to ride in a regular seat. And children under the age of twelve must be in the back seat.
If you live in another state, you can refer to this link http://babyproducts.about.com/od/statecarseatlaws/State_Car_Seat_Laws_Child_Safety_Seat_and_Seatbelt_Laws_by_State.htm to find out what your state's car seat laws are.
Now, notice that these are the law, but research suggests that these are outdated. The recommendation for rear-facing is AT LEAST two-years-old. Further recommendations suggest rear-facing as long as possible.
As for forward-facing, it is recommended that the child stays in a 5-point harness for as long as possible. I will cover how long a child should remain in a booster later in the post.
As for the straps on a 5-point harness seat, the straps should NOT be twisted.
They MUST be tight. Not so tight that the child has indents from them, but they should be snug. The best way to test this is with the pinch test.