Wow, I don't know about the 80 pound thing.
Considering my son didn't even reach five feet till the 8th grade (he's now five foot eight in 11th grade), he really didn't hit the 80 pound mark until probably 6th or 7th grade.
That would have meant he would still have been in his booster seat on his way to the school dance. How sad!
Just kidding, I understand your worries and for the average kid, the 80 pounds may be reasonable but for me, I don't think it would have been fair to put him in a booster seat on his first date and such.

On the plus side, I did stick to the 12 year old air bag rule.