It seems like one of those bizarre laws that should've been changed years ago.
Given there's no limit on how much alcohol they can have at home you could conceivably give your DC as much alcohol as you like at 5 as long as they are at home. I can't think of one other thing that you could let your 5 year old kid do at home which has an 18 legal age limit.
AIBU to think this law should be changed?