Although I´m Estonian, its quite difficult to explain the difference for me aswell..
I would say, that although the meaning for both is AGE,
IGA is more general, and VANUS more concrete, depending on the context. I think (I´m not informed, so it´s just a guess) that IGA comes really from the word AEG (meaning TIME).
So you would be talking of person's LIFETIME (ELUIGA), some period of time you could use an object (KASUTUSIGA- time of usage),
an age when a person is a child (LAPSEIGA), in puberty (MURDEIGA) or when he is already mature (KÜPSUSIGA, TÄISKASVANUIGA), you would wish them "that you may LIVE LONG" (PIKKA IGA!) on their birthday..;
but you would be taking of persons age in years (tema VANUS on 16 aastat-he´s 16 yeas old, his age is 16) on a data-sheet for example, or that person is still really active for his age (ta on oma VANUSE KOHTA ikka väga aktiivne).
Normally you can use IGA everywhere, when you can use VANUS (except when you speak of somebody's age in years)...but not vice versa.