Legal minimum is 135cm or 12 years old, many people stop using them earlier, not realising that they are still required. You can be fined if you stop using one before this height/age.
Best practice: In fact between 135-150cm, it highly depends on the car and the child whether you'll get a safe belt fit for them. The lap belt is the important part, it should run over their pelvis, which is a strong bone. If they are too short, then it will be up on their tummy, which is soft and squishy and full of organs that you really don't want crushed against a seatbelt if you had a crash.
Note that for many children at the upper end of booster age, the seatbelt will fit properly if they scoot their bum right to the back of the seat, but their legs are still too short to bend over the edge. As a result, they scoot forward in order to bend their knees to be comfy and this causes the lap belt to come out of place onto their tummy.
So check for whether they can sit with their bum back and legs bent, and whether the belt fits properly. It will likely be different in different cars. If they feel self conscious, a backless booster is better than nothing, but high back (used properly) offers the best protection.
If you are using a high back, check that the headrest is set properly and the diagonal belt is clipped into the shoulder guide, this should then run over the child's shoulder crossing the collarbone. If it's across their arm it's too low, if it's against their neck it's too high.
High back or backless, the lap belt should run under the "arms" of the booster and the diagonal belt should go under on the buckle side too.