I know some people have said they don't like the age thing, that it should be by weight, but I like having both an age and weight guideline, in fact I think there should be a height guideline too. The reason I say this is because I didn't weigh 80 pounds until I was in 8th grade!!! I was more than tall enough, just very skinny. Even though there were no car seat laws then, I can't imagine sitting in a booster until I was 14!
So far, my DS who is 4, has a very similar body type to what I did. I highly doubt he will be in a booster until he's 80 pounds. Right now he's still using the 5-pt. harness in a high back booster because he's only around 30 pounds.
My oldest DD is 7 and weighs close to 50 pounds. She's still in a booster, without the back, but is getting very close to being tall enough to sit in the seat with a seatbelt positioned correctly. Once she is, I have no problem taking her out of the booster and just letting her use the seatbelt. As long as the she can sit comfortably in the seat with the seatbelt where it is supposed to be, why the concern that she's not 80 pounds? If they're tall enough, wouldn't it be more dangerous to remain in the booster if it made the seatbelt go down too far across their body?