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.

3rd percentile for boys at 12 years old. https://www.cdc.gov/growthcharts/cdc-charts.htm

In NSW the guide is the person should be > 145cm which is somewhere between age 9 and 14 for girls.

https://www.transport.nsw.gov.au/roadsafety/parents/child-ca...