Very good question. There's a defence of reasonable parental discipline. If the grounding went beyond that, I'm pretty sure it would be both a criminal and civil offence. This won't be available in the case of older children, but I think it may be possible to rely on a defence of consent in some circumstances (i.e. the 19 year old could ignore his parent's instructions, but then he wouldn't be allowed to live there or his parents would stop giving him money, so he consents to the grounding - this will probably be OK). I imagine the age threshold is 16 and not 18.