Its 150cm (just under 5 ft I think - maybe 4 ft 10) or 12 here in Germany. That makes sense as that is the height at which ordinary seat belts have some chance of fitting properly rather than going across the child's neck etc. It is also a more likely age/ height correlation!
I've never seen a child who looks obviously under 150cm in a car without a booster cushion, and I know 10 and 11 year olds who still use huge high back boosters that look like arm chairs. When dropping 8yo DS1 off for 8th and 9th birthday parties we and every other invitee bring a booster cushion automatically, and some of his friends' parents drop off these enormous high backed, armed booster seats which then don't fit 3 across the back and cause logistical nightmares when trying to transport 7 people in a 7 seater to the party location... :o
DS1 is 145 cm at 8 and a few months so I guess he won't need a booster til 12, but DD still uses one at 10.5 and isn't embarrassed. A few of her friends are over 150cm but many are the same height as her.
DS1 would have been over 135cm at 6 or 7, and outgrew bog standard high backed boosters due to height and wide shoulders, and I worried about whether I should buy one of the massive hugely expensive arm chair things - but we couldn't have fitted 3 of those across the back of the car so it wasn't really an option. Certainly didn't worry about it being embarrassing and can't imagine just putting him and DD in the car without a booster cushion on visits to the UK, and automatically still put them on inflatable "bubble-bum" booster cushions in the UK last summer as the seat belts wouldn't fit properly regardless of the law saying its fine.