Yep, at this point it really depends on the seat that you have. We have the Graco Nautilus 3-in-1. It will hold my daughter in a 5-point harness until she is 65 lbs. Then I can take the harness out & it will be a high-back booster until she is tall enough to no longer need the back of it (holds the shoulder strap of the seatbelt). Then the back comes off and it is just a booster to raise her up. It works as a booster until 100 lbs.
My daughter is 6 1/2 years old, about 43" tall, and about 40 lbs. She'll likely be in the 5-point harness until she is 65 lbs, or until she starts getting picked on!
Just because a law says you CAN move them to the next stage, it doesn't mean you have to. It is actually safer to keep your child in each stage as long as your car seat allows. That's why car seats are being made to hold children up to 2 years old REAR facing, and keeping them in the 5-point harness at a higher weight. There are plenty of scary videos and stories about children who were buckled in according to law, but still seriously injured or killed because they moved up in stages too fast :(