Fact: Cars are built to suit ADULT SIZED bodies. I have never seen an ADULT SIZED 7 year old.
Whilst it is legal to put away your booster at 7, in safety terms it is a big mistake. Here is a very reliable way to tell if your child is big enough to be out of their booster:
Sit them in the car without their booster, up nice and straight against the back of the seat. If their knees are at the end of the seat and their feet are on the floor – they are ADULT SIZED.
If not, they will usually slump down so their knees reach the end. This means the lap part of the belt is across their soft organs instead of their hips. Alternatively, they will bring their legs up underneath and sometimes sit sideways. Because of their size, the sash part of the belt will be up under their ears instead of across their shoulders. This means they usually tuck it under their arm or behind their back entirely. All of these things can have disastrous consequences, even in a low speed crash.
This is why the 8-12 years range is statistically the most dangerous time in the car – kids are simply not ADULT SIZED. Keep your kids boosted. One day their neck, spine or internal organs might thank you for it.