How do you calculate sodium intake for certain age & size of people?

You don't: 2 grams of sodium is severe restriction. 4 grams of sodium is mild restriction. 6-8 grams is normal. 10 or more is excessive. Since you have no way to measure exactly how much sodium you're eating, there's no point outside of a research lab to try to follow an exact measure of sodium. A good rule of thumb is to avoid adding extra salt at the table if possible and avoid salty foods (like chips).