theory i heard about us legality laws: they're ok with 16 year olds smoking, and 18 year olds drinking. so they made the legal ages 18 and 21, thinking that 21 year olds would buy booze for their 18 year old friends, and 18 year olds would buy 16 year olds cigs, but that neither of those age groups really has friends younger than 18 or 16. kinda makes sense, in a weird way.