I know our law is age 8 and/or 80#. Many people disregard using them past the age of 4. Crab and Yell aren't exactly tiny, but they're not in the 50th percentile for their age either.
Today we were in an accident. A guy ran a red light and I plowed into his drivers' side rear door. The guy on the other side of the light plowed into his passenger side rear quarter panel.
At age 10 1/2, usually to their chagrin, Crab and Yell were in booster seats -- they are 4' 1/2", 63# and 4'4", 61#. Neither one is close enough to 80# to be out of a booster.
Do they hate them? Yes.
Were they kept from injury by the elevation provided? Yes.
Were they thankful they didn't get a cut on their neck (like a classmate of theirs did) from the seatbelt? Yes.
Were they very happy they were in boosters AFTER the wreck? Yes.
My arms and back ache and my bumper is a mess, but we're all okay.
Please, keep your kids in boosters as long as possible.
Edited to add a comment I fully appreciate:
Ohio passed a booster seat law last year, and now (one year later) a fine goes into effect if a child who should be in a booster is not in one. They had a news story on TV about it the other day. A mom whose son has been out of a car seat since age 4 was quoted as saying something along the lines of, "It's so inconvenient." Hmmmm. So would having to go to the hospital every day to visit your brain damaged child if he were in a really bad accident.