If they're too short to fit safely in a seat and use the normal seat belt without one, then yes, a 12-year-old needs a booster seat even if it's not a "child seat".

In the UK the limit is 135cm, which is about the right height for the average 9-year-old, so you're talking about a very small 12-year-old indeed - a good 15cm shorter than average.

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.

In CA AB-435 is written to apply a 5-point fit test to anyone under 16.