High back boosters. What age?

[Flojobunny]

DS is 8 years old, tall for his age but under weight for his height.
He's currently in a high back booster, the head rest is on the highest position.
DP says he's read somewhere that DC shouldn't be in high back booster and he's too old and its dangerous. I got cross with him for putting DS in the front seat without even a usual booster.
I prefer DS having the side and head protection for as long as possible.
Who is right?

children, by law, should be in an appropriate restraint until they are 135cm tall or 12 yrs old, whichever comes first.

my nearly 9 year old is still in a hbb and will stay in for as long as he fits!

To clarify, 135cm or 12 years is when they can legally not use a seat. That doesn't mean they aren't allowed to. Obv it's safest to keep them in one as long as possible

If the seat belt still starts just above where his shoulder is then it's OK. If it had to go up a bit to get over his shoulder it's not.