Every state has their own law but regardless of the law, there is also what is called "best practice". What is legal and what is best are not synomonous. Laws are the MINIMUM that is required, best practice is safest.
Example, the law may say that a child needs to be in some kind of a child restrainst seat according to manufacturer's specifications and my 3 yr old could be in a small booster seat when she was only 35 lbs as long as the seat was rated for 35 lbs. She was not big enough or mature enough in my opinion so she remained in her carseat (rated for 40 lbs). Best practices says at least 40 lbs or 43" (typically 4 yrs old) before they move to a booster. She did have to move to a high-back belt-positioning booster in my husband's truck because his carseat was rated to 35 lbs (exceeding height/weight restrictions is dangerous).
For moving out of booster seats, I believe that best practice is a minimum of 65 lb or 4'9" (usually around age 8) but if your booster seat has not passed its expiration date and is rated above 65 lbs (some are rated to 80-100 lbs) then it is always best to allow them to continue to use until they reach both the pounds and height maximums that the seat is rated for. Once you move them from the booster seat, the knee should bend at the bend of the car's seat and the seat belt should fit properly (if it doesn't, she is not big enough to safely go without a booster seat).
I found this link for you so you don't have to take my word. It is for the National Highway Traffic Safety Administration: