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 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.