Why would additives effect a three year old more than an eight year old?
It depends largely on the context (what additives in what? I'm assuming you're asking about something like medication or ingredients in a specific food), but generally I'd assume it has to do with body size and the capacity of the individual to metabolize the substance. It's sort of like how a person who only weighs 105lbs is much more affected by alcohol than someone who weighs 220lbs.
