I've observed that a whole lot of people absolutely do not keep putting kids in car seats once they're about the height & weight of a petite adult (us included)

I've observed that a whole lot of people absolutely do not keep putting kids in car seats once they're about the height & weight of a petite adult (us included). So the guidelines say one thing, but I'd be surprised if a majority of parents are still putting their kids in them even by age 10, let alone 12.